[libc++] Add incomplete availability markup for std::pmr

This fixes rdar://110330781, which asked for the feature-test macro
for std::pmr to take into account the deployment target. It doesn't
fix h

[libc++] Add incomplete availability markup for std::pmr

This fixes rdar://110330781, which asked for the feature-test macro
for std::pmr to take into account the deployment target. It doesn't
fix https://llvm.org/PR62212, though, because the availability markup
itself must be disabled until some Clang bugs have been fixed.

This is pretty vexing, however at least everything should work once
those Clang bugs have been fixed. In the meantime, this patch at least
adds the required markup (as disabled) and ensures that the feature-test
macro for std::pmr is aware of the deployment target requirement.

Differential Revision: https://reviews.llvm.org/D135813

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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++] Move preferred_name declarations into the forward declaring headers and add pmr preferred names

We currently define the preferred names in multiple places. `basic_string` and `basic_string_

[libc++] Move preferred_name declarations into the forward declaring headers and add pmr preferred names

We currently define the preferred names in multiple places. `basic_string` and `basic_string_view` also have a lot of aliases, which makes the declarations quite long. So let's only add the preferred names in forward-declaring headers to make the implementation more readable and have all the preferred names in one place.

Reviewed By: ldionne

Spies: EricWF, krytarowski, libcxx-commits

Differential Revision: https://reviews.llvm.org/D135824

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[libc++] Add the C++17 <memory_resource> header (mono-patch)

This patch is the rebase and squash of three earlier patches.
It supersedes all three of them.

- D47111: experimental monotonic_buffer_r

[libc++] Add the C++17 <memory_resource> header (mono-patch)

This patch is the rebase and squash of three earlier patches.
It supersedes all three of them.

- D47111: experimental monotonic_buffer_resource.
- D47358: experimental pool resources.
- D47360: Copy std::experimental::pmr to std::pmr.

The significant difference between this patch and the-sum-of-those-three
is that this patch does not add `std::experimental::pmr::monotonic_buffer_resource`
and so on. This patch simply adds the C++17 standard facilities, and
leaves the `std::experimental` namespace entirely alone.

Differential Revision: https://reviews.llvm.org/D89057

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