[clangd] Add ClangdUnit diagnostics tests using annotated code.

Summary:
This adds checks that our diagnostics emit correct ranges in a bunch of cases,
as promised in D41118.

The diagnostics-preamble test is also converted and extended to be a little more
precise.

diagnostics.test stays around as the smoke test for this feature.

Reviewers: ilya-biryukov

Subscribers: klimek, mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D41454

llvm-svn: 323448