This permits using the <locale> header with newlib or picolibc when
LIBCXX_ENABLE_WIDE_CHARACTERS=FALSE.
Since D136682 the __support/xlocale headers themselves include the
headers they require, respecting the LIBCXX_ENABLE_WIDE_CHARACTERS
option.
Differential Revision: https://reviews.llvm.org/D136947
We do ship those headers, so the directory name should not be something
that can potentially conflict with user-defined directories.
Differential Revision: https://reviews.llvm.org/D95956