ccc/Driver: .s defaults to 'assembler-with-cpp' on Darwin.

ccc/Driver: .s defaults to 'assembler-with-cpp' on Darwin.
 - <rdar://problem/6669441> ccc doesn't handle assembler-with-cpp
   semantics correctly (but clang supports it)

 - This is sad, because it requires a fairly useless target
   hook. C'est la vie.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67418 91177308-0d34-0410-b5e6-96231b3b80d8