CodeGen: mark ObjC cstring literals as constant

CodeGen: mark ObjC cstring literals as constant

These strings are constants, mark them as such.  This doesn't matter too much in
practice on MachO since the constants are placed into a special section and not
referred to directly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281854 91177308-0d34-0410-b5e6-96231b3b80d8