Move the functionality to mark all vtables of key functions as used within

a translation unit to the ActOnEndOfTranslationUnit function instead of doing
it at the start of DefineUsedVTables. The latter is now called *recursively*
during template instantiation, which causes an absolutely insane number of
walks of every record decl in the translation unit.

After this patch, an extremely template instantiation heavy test case's compile
time drops by 10x, and we see between 15% and 20% improvement in average
compile times across a project. This is just recovering a regression, it
doesn't make anything faster than it was several weeks ago.

llvm-svn: 121644