[libc++] Clean up and update deployment target features (#96312)

This patch removes many annotations that are not relevant anymore since
we don't support or test back-deploying to macOS < 10.13. It

[libc++] Clean up and update deployment target features (#96312)

This patch removes many annotations that are not relevant anymore since
we don't support or test back-deploying to macOS < 10.13. It also cleans
up raw usage of target triples to identify versions of dylibs shipped on
prior versions of macOS, and uses the target-agnostic Lit features
instead. Finally, it reorders both the Lit backdeployment features and
the corresponding availability macros in the library in a way that makes
more sense, and reformulates the Lit backdeployment features in terms of
when a version of LLVM was introduced instead of encoding the system
versions on which it hasn't been introduced yet. Although one can be
derived from the other, encoding the negative form is extremely
error-prone.

Fixes #80901

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[libc++] Fix deployment target Lit features (#94791)

We were not making any distinction between e.g. the "Apple-flavored"
libc++ built from trunk and the system-provided standard library on
Apple

[libc++] Fix deployment target Lit features (#94791)

We were not making any distinction between e.g. the "Apple-flavored"
libc++ built from trunk and the system-provided standard library on
Apple platforms. For example, any test that would be XFAILed on a
back-deployment target would unexpectedly pass when run on that
deployment target against the tip of trunk Apple-flavored libc++. In
reality, that test would be expected to pass because we're running
against the latest libc++, even if it is Apple-flavored.

To solve this issue, we introduce a new feature that describes whether
the Standard Library in use is the one provided by the system by
default, and that notion is different from the underlying standard
library flavor. We also refactor the existing Lit features to make a
distinction between availability markup and the library we're running
against at runtime, which otherwise limit the flexibility of what we can
express in the test suite. Finally, we refactor some of the
back-deployment versions that were incorrect (such as thinking that LLVM
10 was introduced in macOS 11, when in reality macOS 11 was synced with
LLVM 11).

Fixes #82107

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[libc++] Fix UTF-8 decoding in codecvts (#68442)

This patch fixes one case where the decoding member function `in()` was
returning `partial` instead of `error`. Additionally, it adds large
testsui

[libc++] Fix UTF-8 decoding in codecvts (#68442)

This patch fixes one case where the decoding member function `in()` was
returning `partial` instead of `error`. Additionally, it adds large
testsuite that tests all `codecvt` facets that were added in C++11 and
in C++20. The testsuite covers this bug.

Fixes #60177.

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