ICU-22767 Fix GCC warning and turn warning to errors #2964

Job	Run time
clang-valgrind-intltest (utility)	6m 35s
clang-valgrind-intltest (normalize)	6m 49s
clang-valgrind-intltest (collator)	4m 55s
clang-valgrind-intltest (regex)	5m 20s
clang-valgrind-test	9m 59s
clang-valgrind-intltest (format)	16m 0s
clang-valgrind-intltest (translit)	5m 46s
clang-valgrind-intltest (rbbi)	7m 27s
clang-valgrind-intltest (rbnf)	6m 17s
clang-valgrind-intltest (rbnfrt)	6m 27s
clang-valgrind-intltest (icuserv)	5m 9s
clang-valgrind-intltest (idna)	5m 23s
clang-valgrind-intltest (convert)	6m 34s
clang-valgrind-intltest (rbnfp)	5m 1s
clang-valgrind-intltest (csdet)	5m 14s
clang-valgrind-intltest (spoof)	5m 4s
clang-valgrind-intltest (bidi)	5m 52s
	1h 53m 52s

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