On x86 long double is 80 bit with padding, which produces
uninitialized bits in the result.
This will trigger errors with -fsanitize-memory-param-retval.
Can be triggered with D123979.
Reviewed By: #libc, ldionne
Differential Revision: https://reviews.llvm.org/D123980
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
MSVC targets also have a 64 bit long double, as do MinGW targets on ARM.
This hasn't been noticed in CI because the MSVC configurations there run
with _LIBCPP_HAS_NO_INT128 defined.
This avoids assuming that either __int128_t or double is equal in size to
long double. i386 MinGW targets have sizeof(long double) == 10, which
doesn't match any of the tested types.
Differential Revision: https://reviews.llvm.org/D111671
