Split locale management out of locale_win32. NFCI

For the locale refactor, the locale management functions (newlocale,
freelocale, uselocale) are needed in a separate header from the various _l
functions. This is because some platforms implement the _l functions in terms
of a locale switcher RAII helper, and the locale switcher RAII helper needs
the locale management functions. This patch helps pave the way by getting all
the functions in the right files, so that later diffs aren't completely
horrible.

Unfortunately, the Windows, Cygwin, and MinGW builds seemed to have
bit-rotted, so I wasn't able to test this completely. I don't think I made
things any worse than they already are though.

http://reviews.llvm.org/D17419

llvm-svn: 263020