[clangd] Fix implicit template instatiations appearing as topLevelDecls.

Summary: The parser gives implicit template instantiations to the action's HandleTopLevelDecls callback. This makes semantic highlighting highlight these templated functions multiple times. Fixed by filtering on if a Decl is an implicit function/variable/class instantiation. Also added a testcase to semantic highlighting on this.

Reviewers: hokein, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65510

llvm-svn: 368261