Fix and speedup __libcpp_locale_guard on Windows
The old implementation assumed the POSIX setlocale() API where the old
locale is returned. On Windows, the _new_ locale is returned. This meant
that __libcpp_locale_guard wasn't resetting the locale on destruction.
The new implementation fixes the above issue and takes advantage of
setlocale(LC_ALL) to reduce the number of calls, and also avoids setting
the locale at all if it's not necessary.
Differential Revision: https://reviews.llvm.org/D59572