Support new AST nodes:
Refactor error handling in ImportTemplateArgumentLoc() method.
Add a test for inline namespaces.
Please take a look at FIXMEs I left in some places where I'm not sure that I have chosen a correct solution.
Also, I have some doubts for duplicating NamespaceAliasDecls. I found presence of such decls harmless but maybe I'm not correct here. I left NOTE comments both in the patch and in the tests for it.