[clangd] Initial clang-tidy diagnostics support.

Summary:
This runs checks over a restricted subset of the TU:
 - preprocessor callbacks just receive the truncated PP events that
   occur when a preamble is used.
 - ASTMatchers run only over the top-level decls in the main-file

This patch just turns on one simple check (bugprone-sizeof-expression)
with no configuration. Configuration is complex enough to warrant a separate patch

This depends on a patch allowing traversal to be restricted to a scope.

Reviewers: hokein

Subscribers: srhines, mgorny, ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D54204

llvm-svn: 347036