[clangd] simplify ClangdLSPServer by private-inheriting callback interfaces. NFC

Summary:
There doesn't seem to be any real separation between the current three objects.
Feel free to reject this if you find the current style valuable, though.

(Mostly I'm just looking around for cleanups to help me understand the code).

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38414

llvm-svn: 314587