[libc++][RFC] Always define internal feature test macros (#89178)

Currently, the library-internal feature test macros are only defined if
the feature is not available, and always have the prefix
`

[libc++][RFC] Always define internal feature test macros (#89178)

Currently, the library-internal feature test macros are only defined if
the feature is not available, and always have the prefix
`_LIBCPP_HAS_NO_`. This patch changes that, so that they are always
defined and have the prefix `_LIBCPP_HAS_` instead. This changes the
canonical use of these macros to `#if _LIBCPP_HAS_FEATURE`, which means
that using an undefined macro (e.g. due to a missing include) is
diagnosed now. While this is rather unlikely currently, a similar change
in `<__configuration/availability.h>` caught a few bugs. This also
improves readability, since it removes the double-negation of `#ifndef
_LIBCPP_HAS_NO_FEATURE`.

The current patch only touches the macros defined in `<__config>`. If
people are happy with this approach, I'll make a follow-up PR to also
change the macros defined in `<__config_site>`.

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[libc++][NFC] Increase consistency for namespace closing comments

[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][NFC] Introduce `_LIBCPP_ASSERT_UNCATEGORIZED`.

Replace most uses of `_LIBCPP_ASSERT` with
`_LIBCPP_ASSERT_UNCATEGORIZED`.

This is done as a prerequisite to introducing hardened

[libc++][hardening][NFC] Introduce `_LIBCPP_ASSERT_UNCATEGORIZED`.

Replace most uses of `_LIBCPP_ASSERT` with
`_LIBCPP_ASSERT_UNCATEGORIZED`.

This is done as a prerequisite to introducing hardened mode to libc++.
The idea is to make enabling assertions an opt-in with (somewhat)
fine-grained controls over which categories of assertions are enabled.
The vast majority of assertions are currently uncategorized; the new
macro will allow turning on `_LIBCPP_ASSERT` (the underlying mechanism
for all kinds of assertions) without enabling all the uncategorized
assertions (in the future; this patch preserves the current behavior).

Differential Revision: https://reviews.llvm.org/D153816

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[libc++][filesystem] Avoid using anonymous namespaces in support headers

This avoids using anonymous namespaces in headers and ensures that
the various helper functions get deduplicated across the T

[libc++][filesystem] Avoid using anonymous namespaces in support headers

This avoids using anonymous namespaces in headers and ensures that
the various helper functions get deduplicated across the TUs
implementing <filesystem>. Otherwise, we'd get a definition of
these helper functions in each TU where they are used, which is
entirely unnecessary.

Differential Revision: https://reviews.llvm.org/D152378

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[libc++] Split sources for <filesystem>

The operations.cpp file contained the implementation of a ton of
functionality unrelated to just the filesystem operations, and
filesystem_common.h contained

[libc++] Split sources for <filesystem>

The operations.cpp file contained the implementation of a ton of
functionality unrelated to just the filesystem operations, and
filesystem_common.h contained a lot of unrelated functionality as well.

Splitting this up into more files will make it possible in the future
to support parts of <filesystem> (e.g. path) on systems where there is
no notion of a filesystem.

Differential Revision: https://reviews.llvm.org/D152377

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