CodeGen: try harder to make the CFString structure RW

CodeGen: try harder to make the CFString structure RW

The previous change was insufficient to mark the content as read-write as the
structure itself was marked constant.  Adjust this and add tests to ensure that
the section is marked appropriately as being read-write.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@277200 91177308-0d34-0410-b5e6-96231b3b80d8