Fix two issues with the substitution of template template parameters

when instantiating the declaration of a member template:
  - Only check if the have a template template argument at a specific position
  when we already know that we have template arguments at that level;
  otherwise, we're substituting for a level-reduced template template
  parameter. 
  - When trying to find an instantiated declaration for a template
  template parameter, look into the instantiated scope. This was a
  typo, where we had two checks for TemplateTypeParmDecl, one of
  which should have been a TemplateTemplateParmDecl.

With these changes, tramp3d-v4 passes -fsyntax-only.

llvm-svn: 95421