[PATCH] Avoid extra back reference key lookup in msmangler

[Agustín Bergé]

I think that instead of introducing a new function I should just be using `mangleSourceName`. A template instantiation name is a source name too (considering anonymous type names are treated as source names), it's just that their terminating `'@'` is currently being added at the end of the template argument list. If the argument list no longer appends the `'@'`, then they would be treated as any other source name.

http://reviews.llvm.org/D4130