[clangd] Don't navigate to forward class declaration when go to definition.

Summary:
For some cases, GoToDefinition will navigate to the forward class
declaration, we should always navigate to the class definition.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41661

llvm-svn: 322370