Auto merge of #129716 - compiler-errors:closure-debuginfo, r=cjgillot

Don't use `typeck_root_def_id` in codegen for finding closure's root

Generating debuginfo in codegen currently peels off all the closure-specific generics (which presumably is done because they're redundant). This doesn't currently work correctly for the bodies we synthesize for async closures's returned coroutines (#128506), leading to #129702.

Specifically, `typeck_root_def_id` for some `DefKind::SyntheticCoroutineBody` just returns itself (because it loops while `is_typeck_child` is `true`, and that returns `false` for this defkind), which means we don't end up peeling off the coroutine-specific generics, and we end up encountering an otherwise unreachable `CoroutineWitness` type leading to an ICE.

This PR fixes `is_typeck_child` to consider `DefKind::SyntheticCorotuineBody` to be a typeck child, fixing `typeck_root_def_id` and suppressing this debuginfo bug.

Fixes #129702

Author: bors