add scope to parameters of toUTF32 and toUTFImpl

[aG0aep6G]

For dlang/dmd#12010. Part of an ongoing effort to fix issue 20150.

[dlang-bot]

Thanks for your pull request and interest in making D better, @aG0aep6G! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

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

dub run digger -- build "master + phobos#8109"