These functions weren't added until API 26 (Android 8.0), but libc++ is
supported for API 21 and up.

These APIs are undeclared as of r.android.com/3216959.

This patch adds a large number of missing includes in the libc++ headers
and the test suite. Those were found as part of the effort to move
towards a mostly monolithic top-level std module.

This has been done using the following command

  find libcxx/test -type f -exec perl -pi -e 's|^([^/]+?)((?<!::)size_t)|\1std::\2|' \{} \;

And manually removed some false positives in std/depr/depr.c.headers.

The `std` module doesn't export `::size_t`, this is a preparation for that module.

Reviewed By: ldionne, #libc, EricWF, philnik

Differential Revision: https://reviews.llvm.org/D146088

Reviewed By: ldionne, #libc, EricWF

Spies: EricWF, libcxx-commits

Differential Revision: https://reviews.llvm.org/D121552

Those tests would pass when run on a C Standard Library that actually
provides wide characters, but fail when run on top of one that doesn't.
It's really difficult to test this 100% perfectly in the CI without
introducing an actual platform that doesn't provide these declarations.

Differential Revision: https://reviews.llvm.org/D112937

This allows testing the rest of those headers on most platforms, instead
of XFAILing the whole test just because of a few functions.

As a fly-by fix, remove std/utilities/time/date.time/ctime.pass.cpp,
which was a duplicate of std/language.support/support.runtime/ctime.pass.cpp.

Differential Revision: https://reviews.llvm.org/D108295

Instead of trying to sniff out what features are supported by the
library being tested, the way we normally handle these things is with
Lit annotations. This should not be treated differently.

Differential Revision: https://reviews.llvm.org/D108209

This was added inconsistently in
19fd9039ca242f408493b5c662f9d908eab8555e; Windows doesn't have the
aligned_alloc function (neither MSVC nor MinGW toolchains) and we don't
define _LIBCPP_HAS_ALIGNED_ALLOC while building libcxx.

Differential Revision: https://reviews.llvm.org/D103399

This makes no attempt yet to look into the why/what for each of them,
but makes the CI configuration useful for tracking further regressions.
After looking into each case, they can either be fixed, or converted
into UNSUPPORTED: windows or XFAIL: windows, once the cause is known
and explained.

A number of the filesystem cases can be fixed by patches that are
currently in review.

Differential Revision: https://reviews.llvm.org/D99095

- Several -Wshadow warnings
- Several places where we did not initialize our base class explicitly
- Unused variable warnings
- Some tautological comparisons
- Some places where we'd pass null arguments to functions expecting
  non-null (in unevaluated contexts)
- Add a few pragmas to turn off spurious warnings
- Fix warnings about declarations that don't declare anything
- Properly disable deprecation warnings in ext/ tests (the pragmas we
  were using didn't work on GCC)
- Disable include_as_c.sh.cpp because GCC complains about C++ flags
  when compiling as C. I couldn't find a way to fix this one properly,
  so I'm disabling the test. This isn't great, but at least we'll be
  able to enable warnings in the whole test suite with GCC.