This removes all "TODO: remove these headers" comments from our headers.
Note there seem to be more headers that can be removed, that will be
done in separate commits.
Reviewed By: #libc, ldionne
Differential Revision: https://reviews.llvm.org/D127592
All supported compilers that support C++20 now support concepts. So, remove
`_LIB_LIBCPP_HAS_NO_CONCEPTS` in favor of `_LIBCPP_STD_VER > 17`. Similarly in
the tests, remove `// UNSUPPORTED: libcpp-no-concepts`.
Differential Revision: https://reviews.llvm.org/D121528
Those creep up from time to time. We need to use `int main(int, char**)`
because in freestanding mode, `main` doesn't get special treatment and
special mangling, so we setup a symbol alias from the mangled version of
`main(int, char**)` to `extern "C" main`. That only works if all the tests
are consistent about how they define their main function.
Mark de Wever