Correct importing of the type of a TemplateArgument

It's not clear how this would be tested - I imagine we should have an
ASTImporter test that RAVs the new AST and checks that all the elements
in it are from this ASTContext and not the foreign one... but I know
little about the ASTImporter and how/where that testing might be done.

(post-commit review feedback from Richard Smith on r219900)