PR16519, PR18009: When checking a partial specialization for uses of its own

PR16519, PR18009: When checking a partial specialization for uses of its own
template parameters, don't look for parameters of outer templates. If a problem
is found in a default template argument, point the diagnostic at the partial
specialization (with a note pointing at the default argument) instead of
pointing it at the default argument and leaving it unclear which partial
specialization os problematic.

llvm-svn: 201031