[clangd] Fix SelectionTree traversal of qualified types

Summary:
QualifiedTypeLoc isn't treated like a regular citizen by RecursiveASTVisitor.
This meant we weren't intercepting the traversal of its inner TypeLoc.

Most of the changes here are about exposing kind() so we can improve the
precision of our tests.

This should fix the issue raised in D65067.

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65100

llvm-svn: 366882