Auto merge of #152277 - gurry:114865-ice-format-args, r=chenyukang

Validate source snippet when format input is raw string



Fixes rust-lang/rust#114865

The issue occurred because the user's proc macro respanned the format arg to an unrelated multi-byte string and we ICE'd by landing in the middle of a multi-byte char.

This PR adds validation that prevents the parser from trying to walk such obviously wrong snippets. Such validation already existed for non-raw strings. This PR adds it for raw strings as well.

Author: bors