[clang-tidy] Add diagnostics level to YAML output

Summary:
Also added BuildDirectory for completness and removed unused `Fix`.

Test Plan: check-all

Reviewers: alexfh, gribozavr2

Subscribers: xazax.hun, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D79285