[clangd] Don't run slow clang-tidy checks by default

This uses the fast-check allowlist added in the previous commit.
This is behind a config option to allow users/developers to enable checks
we haven't timed yet, and to allow the --check-tidy-time flag to work.

Fixes https://github.com/clangd/clangd/issues/1337

Differential Revision: https://reviews.llvm.org/D138505