MS ABI: mangleStringLiteral shouldn't rely on the host's endianness

No test case is needed, the one in-tree is sufficient.  The build-bot
never emailed me because something else had upset it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205225 91177308-0d34-0410-b5e6-96231b3b80d8