[libc++][modules] Refactor poisoned_hash_helper (#108296)

The poisoned_hash_helper header was relying on an implicit forward
declaration of std::hash located in <type_traits>. When we improve the

[libc++][modules] Refactor poisoned_hash_helper (#108296)

The poisoned_hash_helper header was relying on an implicit forward
declaration of std::hash located in <type_traits>. When we improve the
modularization of the library, that causes issues, in addition to being
a fundamentally non-portable assumption in the test suite.

It turns out that the reason for relying on a forward declaration is to
be able to test that std::hash is *not* provided if we don't include any
header that provides it. But testing that is actually both non-portable
and not really useful.

Indeed, what harm does it make if additional headers provide std::hash
specializations? That would certainly be conforming -- the Standard
never requires an implementation to avoid providing a declaration when a
given header is included, instead it mandates what *must* be provided
for sure. In that spirit, it would be conforming for e.g. `<cstddef>` to
define the hash specializations if that was our desire. I also don't
read https://wg21.link/P0513R0 as going against that statement. Hence,
this patch just removes that test which doesn't carry its weight.

Fixes #56938

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[libc++] Properly guard std::filesystem with >= C++17 (#72701)

<filesystem> is a C++17 addition. In C++11 and C++14 modes, we actually
have all the code for <filesystem> but it is hidden behind a n

[libc++] Properly guard std::filesystem with >= C++17 (#72701)

<filesystem> is a C++17 addition. In C++11 and C++14 modes, we actually
have all the code for <filesystem> but it is hidden behind a non-inline
namespace __fs so it is not accessible. Instead of doing this unusual
dance, just guard the code for filesystem behind a classic C++17 check
like we normally do.

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[libc++] Expand the contents of LIBCXX_ENABLE_FILESYSTEM

Since LIBCXX_ENABLE_FILESYSTEM now truly represents whether the
platform supports a filesystem (as opposed to whether the <filesystem>
librar

[libc++] Expand the contents of LIBCXX_ENABLE_FILESYSTEM

Since LIBCXX_ENABLE_FILESYSTEM now truly represents whether the
platform supports a filesystem (as opposed to whether the <filesystem>
library is provided), we can provide a few additional classes from
the <filesystem> library even when the platform does not have support
for a filesystem. For example, this allows performing path manipulations
using std::filesystem::path even on platforms where there is no actual
filesystem.

rdar://107061236

Differential Revision: https://reviews.llvm.org/D152382

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[libc++] Add missing test for std::hash<std::filesystem::path>

Differential Revision: https://reviews.llvm.org/D149696