Implement UCN support for C string literals (C99 6.4.3) and add some very...

Implement UCN support for C string literals (C99 6.4.3) and add some very basic tests. Chris Goller has graciously offered to write some test to help validate UCN support.

From a front-end perspective, I believe this code should work for ObjC @-strings. At the moment, I believe we need to tweak the code generation for @-strings (which doesn't appear to handle them). Will be investigating.

llvm-svn: 68076