ICU-22767 Fix GCC warning and turn warning to errors #2962

Job	Run time
clang-valgrind-intltest (utility)	1m 8s
clang-valgrind-test	1m 18s
clang-valgrind-intltest (normalize)	1m 17s
clang-valgrind-intltest (collator)	1m 18s
clang-valgrind-intltest (regex)	1m 11s
clang-valgrind-intltest (format)	1m 17s
clang-valgrind-intltest (translit)	1m 9s
clang-valgrind-intltest (rbbi)	1m 18s
clang-valgrind-intltest (rbnf)	1m 17s
clang-valgrind-intltest (rbnfrt)	1m 12s
clang-valgrind-intltest (icuserv)	1m 17s
clang-valgrind-intltest (idna)	1m 18s
clang-valgrind-intltest (convert)	1m 18s
clang-valgrind-intltest (rbnfp)	1m 16s
clang-valgrind-intltest (csdet)	1m 17s
clang-valgrind-intltest (spoof)	1m 9s
clang-valgrind-intltest (bidi)	1m 17s
	21m 17s

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