[libc++] Fix modules builds when features are removed

[libc++] Fix modules builds when features are removed

When some headers are not available because we removed features like
localization or threads, the compiler should not try to include these
headers when building modules. To avoid that from happening, add a
requires-declaration that is never satisfied when the configuration
in use doesn't support a header.

rdar://93777687

Differential Revision: https://reviews.llvm.org/D127127