fix issue 18993 (REG ~master) - toLower is broken for UTF chars

[ghost]

No description provided.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18993	regression	toLower is broken for UTF chars

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "stable + phobos#6586"

[wilzbach]

LGTM. Thanks for the fast PR!

[ghost]

ping, there's a problem with the project tester here.

[wilzbach]

Yep - https://ci.dlang.io is offline.

[Temtaime]

@bbasile would it go to master branch ?

[ghost]

I can try but this must be this is usually done by the release czar or at least a member.

[ghost]

Unfortunately This cant be cherry picked due to a conflict. Don't know if they'll take it (commit hash will be different)

[ghost]

@Temtaime , see #6605