Add a few (currently failing) tests for the PR13207 (template mangling in the...

Add a few (currently failing) tests for the PR13207 (template mangling in the presence of back references).

I've added an extra FileCheck pass for that with an extra "CURRENT" prefix.
I've carefully chosed the CURRENT/CORRECT prefixes so they
a) are self-descriptive
b) have the same length so the mangling between the current and the correct version is obvious

Feel free to ask me to change the prefixes if you know a better alternative.

llvm-svn: 159250