CodeGen: mark ObjC cstring literals as unnamed_addr

CodeGen: mark ObjC cstring literals as unnamed_addr

These are all emitted into a section with a cstring_literal attribute.  The
attribute permits the linker to coalesce the string contents.  The address of
the strings are not important.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281855 91177308-0d34-0410-b5e6-96231b3b80d8