Add CXXFLAGS back to the Link command.

This is essentially reverting one piece of 184793 to try to fix one of Apple's
buildbots.  I will check with Eric to see if this is OK or if we need to find
some other solution.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185060 91177308-0d34-0410-b5e6-96231b3b80d8