Reland r219810 "Fix late template parsing leak with incremental processing"

Original message:
Add a second late template parser callback meant to cleanup any
resources allocated by late template parsing.  Call it from the
Sema::ActOnEndOfTranslationUnit method after all pending template
instantiations have been completed.  Teach Parser::ParseTopLevelDecl to
install the cleanup callback when incremental processing is enabled so
that Parser::TemplateIds can be freed.

Patch by Brad King!