Before checking a template template argument against its corresponding

template template parameter, substitute any prior template arguments
into the template template parameter. This, for example, allows us to
properly check the template template argument for a class such as:

  template<typename T, template<T Value> class X> struct Foo;

The actual implementation of this feature was trivial; most of the
change is dedicated to giving decent diagnostics when this
substitution goes horribly wrong. We now get a note like:

  note: while substituting prior template arguments into template
      template parameter 'X' [with T = float]

As part of this change, enabled some very pedantic checking when
comparing template template parameter lists, which shook out a bug in
our overly-eager checking of default arguments of template template
parameters. We now perform only minimal checking of such default
arguments when they are initially parsed.

llvm-svn: 86864