Re-apply "Revert r166370 and r166540 now that Xcode 4.6 has been available for a while."

This reverts commit r176892.

I had reverted this a while back to give Chromium more time to update, and
Nico says it should be OK now.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@223108 91177308-0d34-0410-b5e6-96231b3b80d8