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

Those changes were added as a temporary workaround for Xcode 4.5 passing the
-Wno-arc-abi option.  Xcode 4.6 does not pass that option so this should no
longer be necessary.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176887 91177308-0d34-0410-b5e6-96231b3b80d8