[clangd] Fix the annotate tweak after rL366893

Summary:
After rL366893, the annoate tweak is not activated when we select the
whole file (the commonAncestor is TUDecl but we intend to return null).

This patch fixes this, and also avoid traversing the TUDecl.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65210

llvm-svn: 366996