Add methods for emitting diagnostics with no location as well as a special diagnostic for configuration errors.
These show up in the errors as [clang-tidy-config].
The reason to use a custom name rather than the check name is to distinguish the error isn't the same category as the check that reported it.
Reviewed By: aaron.ballman
Differential Revision: https://reviews.llvm.org/D91885
This reverts commit 1e0669bfe05f0f48ee88152c4a1d581f484f8d67
(and follow-ups 698a12712920c214e39bb215fe26fad2e099425b and
52bbdad7d63fd060d102b3591b433d116a982255).
The tests fail fail on Windows, see https://reviews.llvm.org/D74669