[libcxx] Implement locale.h to fix modules build

Summary:
Because `locale.h` isn't part of the libc++ modules the class definitions it provides are exported as part of `__locale` (since it happens to be build first). This breaks `<clocale>` which exports `std::lconv` without including `<__locale>`.

This patch implements `locale.h` to fix this issue, it also adds support for testing libc++ with modules.




Reviewers: mclow.lists, rsmith, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D26826

llvm-svn: 287413