Add stable for Constant in smir

[ouz-a]

Previously #114587 we covered much of the groundwork needed to cover Const in smir, so there is no reason keep Constant as String.

r? @spastorino

[rustbot]

This PR changes Stable MIR

cc @oli-obk, @celinval, @spastorino

[spastorino]

@bors r+ rollup

[bors]

📌 Commit cab9fc9 has been approved by spastorino

It is now in the queue for this repository.

[bors]

⌛ Testing commit cab9fc9 with merge 734a0d0...

[bors]

☀️ Test successful - checks-actions
Approved by: spastorino
Pushing 734a0d0 to master...

[rust-timer]

Finished benchmarking commit (734a0d0): 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

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)	-	-	0
Improvements ✅ (primary)	-3.4%	[-3.4%, -3.4%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.4%	[-3.4%, -3.4%]	2

Cycles

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

Binary size

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

Bootstrap: 630.392s -> 631.205s (0.13%)
Artifact size: 315.69 MiB -> 315.72 MiB (0.01%)