[libcxx] [test] Update locale names for Windows.

locale.codecvt.byname/ctor_char.pass.cpp:
This test used to use "en_US" as a plain string instead of using platform_support.
Need to fix this because MS STL expects "en-US" instead.

platform_support.h:
These are the legacy Windows locale names. Should use IETF tags instead.
I've also added en_US, since a test was using that as a locale string as well.

msvc_stdlib_force_include.hpp:
Remove _MSVC_STL_VER. The libraries will directly define _MSVC_STL_VERSION in the future.

Fixes D29351.

llvm-svn: 305000