ICU-22793 Clang-Tidy: google-readability-casting

[roubert]

https://releases.llvm.org/19.1.0/tools/clang/tools/extra/docs/clang-tidy/checks/google/readability-casting.html

Before running clang-tidy this time, I temporarily replaced UBool with bool to make it find superfluous such casts. This PR covers what was not already included in commit 0178a07.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22793
• 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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• icu4c/source/common/ucnvmbcs.cpp is different

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