[clangd] Suggest code-completions for overriding base class virtual methods.

Summary:
Whenever a code-completion is triggered within a class/struct/union looks at
base classes and figures out non-overriden virtual functions. Than suggests
completions for those.

Reviewers: ilya-biryukov, hokein, ioeric

Reviewed By: hokein

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50898

llvm-svn: 340530