CodeGen: further merge cstring literal construction

Use the new CreateCStringLiteral in an additional site.  Now all the C string
literals are created in one function.  Furthermore, mark the additional literal
as an `unnamed_addr constant`.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281997 91177308-0d34-0410-b5e6-96231b3b80d8