fix #124714 str.to_lowercase sigma handling

[Marcondiro]

Hello,
This PR fixes issue #124714 about 'Σ' handling in str.to_lowercase().
The fix consists in considering the full original string during 'Σ' handling instead of considering just the substring left after the optimized ascii handling.
A new test is added to avoid regression.
Thanks!

[rustbot]

r? @joboet

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[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git rebase -i master
$ # delete any merge commits in the editor that appears
$ git push --force-with-lease

The following commits are merge commits:

• 8d1f3ef

[joboet]

@rustbot author

[Marcondiro]

@rustbot review

[joboet]

Very cool, thank you!
@bors r+

[bors]

📌 Commit bbdf972 has been approved by joboet

It is now in the queue for this repository.

[bors]

⌛ Testing commit bbdf972 with merge 238c1e7...

[bors]

☀️ Test successful - checks-actions
Approved by: joboet
Pushing 238c1e7 to master...

[rust-timer]

Finished benchmarking commit (238c1e7): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.4%]	5
All ❌✅ (primary)	-	-	0

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)	-	-	0
Improvements ✅ (secondary)	-3.2%	[-3.2%, -3.2%]	1
All ❌✅ (primary)	-	-	0

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: 674.785s -> 675.773s (0.15%)
Artifact size: 315.96 MiB -> 315.82 MiB (-0.04%)