Auto merge of #97903 - est31:unused_macro_rules_compile_error, r=petrochenkov

Never regard macro rules with compile_error! invocations as unused

The very point of compile_error! is to never be reached, and one of
the use cases of the macro, currently also listed as examples in the
documentation of compile_error, is to create nicer errors for wrong
macro invocations. Thus, we should never warn about unused macro arms
that contain invocations of compile_error.

See also rust-lang/rust#96150 (comment) and the discussion after that.

Furthermore, the PR also contains two commits to silence `unused_macro_rules` when a macro has an invalid rule, and to add a test that `unused_macros` does not behave badly in the same situation.

r? `@petrochenkov` as I've talked to them about this

Author: bors