Add implementations of some MSVC intrinsics

Adds an implementation for _InterlockedCompareExchangePointer() and
__faststorefence().

Patch by David Ziman!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200239 91177308-0d34-0410-b5e6-96231b3b80d8