[libc++][hardening] Categorize more assertions. (#75918)

Also introduce `_LIBCPP_ASSERT_PEDANTIC` for assertions violating which
results in a no-op or other benign behavior, but which may neverthel

[libc++][hardening] Categorize more assertions. (#75918)

Also introduce `_LIBCPP_ASSERT_PEDANTIC` for assertions violating which
results in a no-op or other benign behavior, but which may nevertheless
indicate a bug in the invoking code.

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[libc++][hardening] Rework macros for enabling the hardening mode. (#70575)

1. Instead of using individual "boolean" macros, have an "enum" macro
`_LIBCPP_HARDENING_MODE`. This avoids issues with m

[libc++][hardening] Rework macros for enabling the hardening mode. (#70575)

1. Instead of using individual "boolean" macros, have an "enum" macro
`_LIBCPP_HARDENING_MODE`. This avoids issues with macros being
mutually exclusive and makes overriding the hardening mode within a TU
more straightforward.

2. Rename the safe mode to debug-lite.

This brings the code in line with the RFC:
https://discourse.llvm.org/t/rfc-hardening-in-libc/73925

Fixes #65101

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[libc++][hardening] Add back the safe mode.

The safe mode is in-between the hardened and the debug modes, extending
the checks contained in the hardened mode with certain checks that are
relatively

[libc++][hardening] Add back the safe mode.

The safe mode is in-between the hardened and the debug modes, extending
the checks contained in the hardened mode with certain checks that are
relatively cheap and prevent common sources of errors but aren't
security-critical. Thus, the safe mode trades off some performance for
a wider set of checks, but unlike the debug mode, it can still be used
in production.

Differential Revision: https://reviews.llvm.org/D158823

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[libc++][hardening][NFC] Rework the Lit feature for detecting the hardening mode.

Make it a multichoice string to closer mirror the CMake variable. This
allows writing `UNSUPPORTED: libcpp-hardening

[libc++][hardening][NFC] Rework the Lit feature for detecting the hardening mode.

Make it a multichoice string to closer mirror the CMake variable. This
allows writing `UNSUPPORTED: libcpp-hardening-mode=unchecked` rather
than `UNSUPPORTED: !libcpp-has-hardened-mode && !libcpp-has-debug-mode`.

Differential Revision: https://reviews.llvm.org/D155906

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[libc++][hardening] Don't trigger uncategorized assertions in the hardened mode.

The hardened mode is intended to only include security-critical,
relatively low-overhead checks that are intended to

[libc++][hardening] Don't trigger uncategorized assertions in the hardened mode.

The hardened mode is intended to only include security-critical,
relatively low-overhead checks that are intended to be usable in
production. By default, assertions are excluded from this mode.

Differential Revision: https://reviews.llvm.org/D155866

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[libc++][hardening] Deprecate `_LIBCPP_ENABLE_ASSERTIONS`.

`_LIBCPP_ENABLE_ASSERTIONS` was used to enable the "safe" mode in
libc++. Libc++ now provides the hardened mode and the debug mode that
rep

[libc++][hardening] Deprecate `_LIBCPP_ENABLE_ASSERTIONS`.

`_LIBCPP_ENABLE_ASSERTIONS` was used to enable the "safe" mode in
libc++. Libc++ now provides the hardened mode and the debug mode that
replace the safe mode.

For backward compatibility, enabling `_LIBCPP_ENABLE_ASSERTIONS` now
enables the hardened mode. Note that the hardened mode provides
a narrower set of checks than the previous "safe" mode (only
security-critical checks that are performant enough to be used in
production).

Differential Revision: https://reviews.llvm.org/D154997

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[libc++] Use named Lit features to flag back-deployment XFAILs

Instead of writing something like `XFAIL: use_system_cxx_lib && target=...`
to XFAIL back-deployment tests, introduce named Lit feature

[libc++] Use named Lit features to flag back-deployment XFAILs

Instead of writing something like `XFAIL: use_system_cxx_lib && target=...`
to XFAIL back-deployment tests, introduce named Lit features like
`availability-shared_mutex-missing` to represent those. This makes the
XFAIL annotations leaner, and solves the problem of XFAIL comments
potentially getting out of sync. This would also make it easier for
another vendor to add their own annotations to the test suite by simply
changing how the feature is defined for their OS releases, instead
of having to modify hundreds of tests to add repetitive annotations.

This doesn't touch *all* annotations -- only annotations that were widely
duplicated are given named features (e.g. when filesystem or shared_mutex
were introduced). I still think it probably doesn't make sense to have a
named feature for every single fix we make to the dylib.

This is in essence a revert of 2659663, but since then the test suite
has changed significantly. Back when I did 2659663, the configuration
files we have for the test suite right now were being bootstrapped and
it wasn't clear how to provide these features for back-deployment in
that context. Since then, we have a streamlined way of defining these
features in `features.py` and that doesn't impact the ability for a
configuration file to stay minimal.

The original motivation for this change was that I am about to propose
a change that would touch essentially all XFAIL annotations for back-deployment
in the test suite, and this greatly reduces the number of lines changed
by that upcoming change, in addition to making the test suite generally
better.

Differential Revision: https://reviews.llvm.org/D146359

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[libc++] Improve the implementation of std::unreachable

First, use __builtin_unreachable unconditionally. It is implemented by
all the compilers that we support. Clang started supporting it around
C

[libc++] Improve the implementation of std::unreachable

First, use __builtin_unreachable unconditionally. It is implemented by
all the compilers that we support. Clang started supporting it around
Clang 4, and GCC around GCC 4.10.

Also add _LIBCPP_ASSERT so that we will actually get a guaranteed crash
if we reached `std::unreachable()` and assertions have been enabled,
since that's UB that's extremely easy to catch.

Differential Revision: https://reviews.llvm.org/D131620

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