ICU-22873 Update CI to clang18 and enable -Werror #2801

Job	Run time
clang-valgrind-test	11m 17s
clang-valgrind-intltest (utility)	6m 59s
clang-valgrind-intltest (normalize)	6m 25s
clang-valgrind-intltest (collator)	5m 58s
clang-valgrind-intltest (regex)	5m 47s
clang-valgrind-intltest (format)	16m 23s
clang-valgrind-intltest (translit)	6m 18s
clang-valgrind-intltest (rbbi)	7m 46s
clang-valgrind-intltest (rbnf)	6m 33s
clang-valgrind-intltest (rbnfrt)	6m 30s
clang-valgrind-intltest (icuserv)	5m 11s
clang-valgrind-intltest (idna)	5m 11s
clang-valgrind-intltest (convert)	6m 44s
clang-valgrind-intltest (rbnfp)	5m 17s
clang-valgrind-intltest (csdet)	5m 24s
clang-valgrind-intltest (spoof)	5m 40s
clang-valgrind-intltest (bidi)	5m 30s
	1h 58m 53s

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