Default to use libc++ on OS X 10.9+ and iOS 7+.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193934 91177308-0d34-0410-b5e6-96231b3b80d8
OS X 10.9+ and iOS 7+ support load/store of big atomic objects.

rdar://13973577

Patch by Fariborz Jahanian.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193935 91177308-0d34-0410-b5e6-96231b3b80d8