Revert r194097: "With this patch -Wwrite-strings is still implemented with the terrible
    hack of passing -fconst-strings to -cc1"

Passing or not a language option based on diagnostic settings is a bad idea, it breaks
using a PCH that was compiled with different diagnostic settings.

Also add a test case to make sure we don't regress.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@200964 91177308-0d34-0410-b5e6-96231b3b80d8