Add implementations of the MSVC barrier intrinsics

Summary:
These are deprecated in VS 2012 according to MSDN.  They don't actually
compile down to any code.  They prevent the compiler from reordering
memory accesses across the barrier, which is what a memory-clobbering
volatile asm does.

Reviewers: echristo

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1954

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192860 91177308-0d34-0410-b5e6-96231b3b80d8
Add the context that a function was created in as the context for the
function, not the context of the context.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192862 91177308-0d34-0410-b5e6-96231b3b80d8