ICU-22356 Revert ICU-21833 change for ures.h

[xry111]

These usages of UChar are in inline functions for calling C API functions, so they should not be changed to char16_t or the code will fail to compile with -DUCHAR_TYPE=.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22356
• Required: The PR title must be prefixed with a JIRA Issue number.
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• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

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[roubert]

The code here is explicitly C++ code, which is why I think the solution proposed in PR #2432 is more in line with the original intentions behind ICU-21833.

[xry111]

The code here is explicitly C++ code, which is why I think the solution proposed in PR #2432 is more in line with the original intentions behind ICU-21833.

IMO #2432 is an align rule break and leads to undefined behavior. Or am I misunderstanding something here?

[roubert]

IMO #2432 is an align rule break and leads to undefined behavior.

To ensure that this doesn't become a problem, I updated that PR to instead use the ICU4C helper class ConstChar16Ptr to do the required typecasting in a safe way.

[xry111]

Closing as superseded.