[PATCH] Implement substitutions of placeholders in the replacement text of the fixits.

[Daniel Jasper]

I am not 100% sure the additional code and maintenance costs here pay for the added benefit. Some random thoughts:
* Simply replacing predefined variables is powerful in some ways, but might be insufficient for other configurations things we might want to do. Maybe we should instead consider adapting the system that creates/formats Clang's diagnostics.
* Added the right username in a TODO comment seems to be a really special case. First, it seems the only thing I can come up with that cannot be easily determine when the actual check is executed. Second, the value of adding the right user name (or of this check having an automated fix at all) seems relatively small.

I'd like to think some more (and maybe get additional impact), before going forward with this patch as implemented.

http://reviews.llvm.org/D5408