Revert "[analyzer][NFC] Don't allow dependency checkers to emit diagnostics"

Summary:
This reverts commit 33fb9cbe.

That commit violates layering by adding a dependency from StaticAnalyzer/Core
back to StaticAnalyzer/FrontEnd, creating a circular dependency.

I can't see a clean way to fix it except refactoring.

Reviewers: echristo, Szelethus, martong

Subscribers: xazax.hun, baloghadamsoftware, szepet, rnkovacs, a.sidorin, mikhail.ramalho, donat.nagy, dkrupp, Charusso, ASDenysPetrov, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81752