Remove unnecessary indirection around hir::ConstArg

[JonathanBrouwer]

Probably doesn't have measurable perf effect because ConstArg is not that common of a type (yet), but it's nicer this way regardless imo :)

[rustbot]

HIR ty lowering was modified

cc @fmease

[rustbot]

r? @mu001999

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• Owners of files modified in this PR: ast_lowering, compiler
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[JonathanBrouwer]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: db44544 (db4454453529bb925d18dff1b42fe89207b4d941)
Base parent: 92f3cfc (92f3cfc6887f3dd2257504d6f75c2d377c38ea89)

[rust-timer]

Finished benchmarking commit (db44544): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

This perf run didn't have relevant results for this metric.

Cycles

Results (secondary -2.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.8%	[2.5%, 5.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-13.7%	[-13.7%, -13.7%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%, secondary 0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.1%]	4
Regressions ❌ (secondary)	0.0%	[0.0%, 0.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.1%]	4

Bootstrap: 488.853s -> 490.569s (0.35%)
Artifact size: 406.48 MiB -> 406.62 MiB (0.04%)