[clangd] Replaced WorkerRequest with std::function...

Summary:
And implemented a helper function to dump an AST of a file for
testing/debugging purposes.

Reviewers: bkramer, krasimir

Reviewed By: krasimir

Subscribers: klimek, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D33415

llvm-svn: 303634