[clangd] Add ClangdServer::customAction() extension point

This lets basic AST-consuming actions be defined outside clangdserver.
(it essentially exposes TUScheduler::runWithAST).

Two use cases for now:
 - replace ClangdServer::dumpAST, which doesn't really belong in the public
   interface (a followup patch will do this)
 - allow embedders to add/experiment with extra features
   (e.g. I know some who want to try crazy AST+ML code completion...)

Maybe in the future we should use this sort of mechanism to make ClangdServer
less of a monolith, but that's not in scope for now.
This would probably require a richer and more carefully-thought-out API.

Differential Revision: https://reviews.llvm.org/D85883

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