Extend the notion of active template instantiations to include the

context of a template-id for which we need to instantiate default
template arguments.

In the TextDiagnosticPrinter, don't suppress the caret diagnostic if
we are producing a non-note diagnostic that follows a note diagnostic
with the same location, because notes are (conceptually) a part of the
warning or error that comes before them.

llvm-svn: 66572