[PATCH] Implement __readeflags and __writeeflags intrinsics

[Reid Kleckner]

  OK.  I initially pushed to make Intrin.h be Windows-only because I didn't
  want to give the impression that it was Clang's blessed intrinsics
  interface, but we could revisit that.  Feel free to send a patch to remove
  the #ifndef _MSC_VER #include_next hack.

  I don't think these should go in immintrin.h unless Intel documents these
  intrinsics as belonging there.

http://llvm-reviews.chandlerc.com/D2468