Partially revert r141374: allow use of libc++ prior to Mac OS X 10.7.

Apple only supports libc++ on OS X 10.7 and later but for the open-source
compiler that restriction doesn't make sense.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@141566 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed