[clangd] Add CMake option to (not) link in clang-tidy checks

This reduces the size of the dependency graph and makes incremental
development a little more pleasant (less rebuilding).

This introduces a bit of complexity/fragility as some tests verify
clang-tidy behavior. I attempted to isolate these and build/run as much
of the tests as possible in both configs to prevent rot.

Expectation is that (some) developers will use this locally, but
buildbots etc will keep testing clang-tidy.

Fixes https://github.com/clangd/clangd/issues/233

Differential Revision: https://reviews.llvm.org/D105679