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

This reverts commit 176887.  Nico asked for more time to move to Xcode 4.6.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176892 91177308-0d34-0410-b5e6-96231b3b80d8