[ASTImporter] Import the default argument of TemplateTemplateParmDecl

Same idea as in D92103 and D92106, but I realised after creating those reviews that there are
also TemplateTemplateParmDecls that can have default arguments, so here's hopefully the
last patch for default template arguments.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D92119