compiler: add `ExternAbi::is_rustic_abi` Various parts of the compiler were hand-rolling this extremely simple check that is nonetheless easy to get wrong as the compiler evolves over time. Discourage them from being so "original" again by replacing it with a single implementation on the type that represents these ABIs. This simplifies a surprising amount of code as a result. Also fixes #132981, an ICE that emerged due to other checks being made stricter.