[libc++][Modules] locale fails to compile with clang modules when...

[libc++][Modules] locale fails to compile with clang modules when _LIBCPP_LOCALE__L_EXTENSIONS is undefined

When `__locale_dir/locale_base_api/locale_guard.h is` compiled independently, as it is when it's in its own clang module, it fails to compile due to `locale_t` being undefined. It needs to include `__locale` to get that, instead of just `clocale`.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D158669

(cherry picked from commit 6021c78f)