Nuke getGlobalContext() from LLVM (but the C API)

The only use for getGlobalContext() is in the C API.
Let's just move the static global here and nuke the C++ API.

Differential Revision: http://reviews.llvm.org/D19094

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266380