Re-landing a cleaned up implementation of r284550

This time it should actually work. The previous implementaiton was not
getting the linker or compiler flag set correctly in all the right
situations. By moving the check down and basing it of whether or not CXX
is set I we can have the logic to add the flags exist only once for the
linker and once for the compiler instead of duplicating it.

llvm-svn: 284756