[clangd] Highlighting auto variables as the deduced type.
Done in VisitDeclaratorDecl as the AutoTypeLoc is not deduced.
Scoped to only work for variables.
auto function return values need to be handled in a special way (separate patch for that).
auto's that are in lambdas ar enot highlighted as we don't highlight their underlying type (it's a RecordDecl, but the name is not an identifier so it returns in addToken).
Reviewers: hokein, ilya-biryukov
Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits
Differential Revision: https://reviews.llvm.org/D65996