[clang-tidy] Add support for diagnostics with no location

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