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

[compiler-errors]

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

[rustbot]

r? @fmease

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[rustbot]

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

[fmease]

I lack the necessary familiarity and context
r? cjgillot (#128506)

[compiler-errors]

@cjgillot: Are you available to review this, or should I re-roll?

[cjgillot]

I'll get to it this weekend, sorry for the delay.

[compiler-errors]

@rustbot ready

[cjgillot]

@bors r+ rollup

[bors]

📌 Commit 63405fc has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit 63405fc with merge 13b5a4e...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 13b5a4e to master...

[rust-timer]

Finished benchmarking commit (13b5a4e): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (primary -2.6%, secondary -0.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.7%	[2.7%, 2.7%]	1
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	-2.6%	[-2.6%, -2.6%]	1

Cycles

Results (primary 1.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.5%	[1.5%, 1.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.5%	[1.5%, 1.5%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 760.015s -> 759.454s (-0.07%)
Artifact size: 341.34 MiB -> 341.32 MiB (-0.01%)