ICU-22873 Update CI to clang18 and enable -Werror

[rp9-next]

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22873
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[jira-pull-request-webhook]

Hooray! The files in the branch are the same across the force-push. 😃

~ Your Friendly Jira-GitHub PR Checker Bot

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• .github/workflows/icu4c.yml is different

View Diff Across Force-Push

~ Your Friendly Jira-GitHub PR Checker Bot

[roubert]

LGTM

[markusicu]

Hi @rp9-next we normally let team members merge their own pull requests (partly in case they want to look it over themselves and maybe tweak something). Given our time zone difference, do you want us to merge some of your PRs on your behalf when they are approved? If so, maybe note that in the PR description? Or we try to remember which team member is generally ok with that?
@richgillam @echeran @mihnita opinion?

[rp9-next]

Hi @rp9-next we normally let team members merge their own pull requests (partly in case they want to look it over themselves and maybe tweak something). Given our time zone difference, do you want us to merge some of your PRs on your behalf when they are approved? If so, maybe note that in the PR description? Or we try to remember which team member is generally ok with that? @richgillam @echeran @mihnita opinion?

Thanks, I am generally ok for people to merge my PRs (Due to time zone differences, I get time to amend any changes before anyone reviews). I will try to keep a message in description in the future, or perhaps a label (like "ready-to-merge" or "author-approves") ?