Split locale management out of ibm/xlocale.h. NFCI

This is one part of many of a locale refactor. See
http://reviews.llvm.org/D17146 for an idea of where this is going.

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, I have no access to an AIX machine to build with, so this change
has been made blind. Also, the original author (Xing Xue) does not appear to
have a Phabricator account.

Reviewed: http://reviews.llvm.org/D17380
llvm-svn: 261230