ICU-22767 Fix GCC warning and turn warning to errors #2914

Job	Run time
clang-valgrind-test	10m 20s
clang-valgrind-intltest (utility)	6m 40s
clang-valgrind-intltest (normalize)	6m 55s
clang-valgrind-intltest (collator)	5m 25s
clang-valgrind-intltest (regex)	5m 11s
clang-valgrind-intltest (format)	16m 36s
clang-valgrind-intltest (translit)	6m 53s
clang-valgrind-intltest (rbbi)	7m 39s
clang-valgrind-intltest (rbnf)	6m 14s
clang-valgrind-intltest (rbnfrt)	6m 5s
clang-valgrind-intltest (icuserv)	4m 59s
clang-valgrind-intltest (idna)	5m 29s
clang-valgrind-intltest (convert)	7m 5s
clang-valgrind-intltest (rbnfp)	5m 21s
clang-valgrind-intltest (csdet)	5m 30s
clang-valgrind-intltest (spoof)	5m 20s
clang-valgrind-intltest (bidi)	5m 32s
	1h 57m 14s

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