Introduce a new representation for template template

Introduce a new representation for template template
parameters. Rather than storing them as either declarations (for the
non-dependent case) or expressions (for the dependent case), we now
(always) store them as TemplateNames.

The primary change here is to add a new kind of TemplateArgument,
which stores a TemplateName. However, making that change ripples to
every switch on a TemplateArgument's kind, also affecting
TemplateArgumentLocInfo/TemplateArgumentLoc, default template
arguments for template template parameters, type-checking of template
template arguments, etc.

This change is light on testing. It should fix several pre-existing
problems with template template parameters, such as:
  - the inability to use dependent template names as template template
  arguments
  - template template parameter default arguments cannot be
  instantiation

However, there are enough pieces missing that more implementation is
required before we can adequately test template template parameters.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@86777 91177308-0d34-0410-b5e6-96231b3b80d8