Merge LLVM/Clang 3.4 into PNaCl's branch

This fixes up various merge conflicts.

The PNaCl branch of Clang contained various changes cherry-picked from
between 3.3 and 3.4, so these no longer appear as localmods after the
merge.  This includes changes to:

 * atomics
 * C++ method pointers for le32
 * use of pow() intrinsic

TEST=PNaCl toolchain trybots

Review URL:!topic/native-client-reviews/OD-h-xSTCu4