[PATCH] Add implementations of the MSVC barrier intrinsics

[Eric Christopher]

  Yuck.

  OK, I guess, ISTR something that would serve as a barrier other than what you've got, but if we don't expect people to use it a lot then I guess it'll be ok.

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

BRANCH
  barrier-intrinsics

ARCANIST PROJECT
  clang