[AST] Include the TranslationUnitDecl when traversing with TraversalScope

Given `int foo, bar;`, TraverseAST reveals this tree:
  TranslationUnitDecl
   - foo
   - bar

Before this patch, with the TraversalScope set to {foo}, TraverseAST yields:
  foo

After this patch it yields:
  TranslationUnitDecl
  - foo

Also, TraverseDecl(TranslationUnitDecl) now respects the traversal scope.

---

The main effect of this today is that clang-tidy checks that match the
translationUnitDecl(), either in order to traverse it or check
parentage, should work.

Differential Revision: https://reviews.llvm.org/D104071