Some "prep" work for handling ObjC @-string constants that contain UTF-8. No functionality change.

Changed GenerateConstantString() to take an ObjCStringLiteral (instead of a std::string). While this isn't strictly necessary, it seems cleaner and allows us to cache to "containsNonAscii" if necessary (to avoid checking in both Sema and CodeGen).

llvm-svn: 68114