Improve diagnostics when a default template argument does not match

with its corresponding template parameter. This can happen when we
performed some substitution into the default template argument and
what we had doesn't match any more, e.g.,

  template<int> struct A;
  template<typename T, template<T> class X = A> class B;

  B<long> b;

Previously, we'd emit a pretty but disembodied diagnostic showing how
the default argument didn't match the template parameter. The
diagnostic was good, but nothing tied it to the *use* of the default
argument in "B<long>". This commit fixes that.

Also, tweak the counting of active template instantiations to avoid
counting non-instantiation records, such as those we create for
(surprise!) checking default arguments, instantiating default
arguments, and performing substitutions as part of template argument
deduction.

llvm-svn: 86884