[clangd][NFC] Make use of TagDecl inside type for hover on auto

Summary:
We were traversing AST twice to get the Decl in case of sugared
types(auto, decltype). They seem to be same in practice, so this patch gets rid
of the second traversal and makes use of TagDecl inside QualType instead.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71597