ICU-22876 C++ UnicodeSet/USet easy item iteration #2713

Job	Run time
clang-valgrind-test	10m 20s
clang-valgrind-intltest (utility)	6m 44s
clang-valgrind-intltest (normalize)	6m 36s
clang-valgrind-intltest (collator)	5m 5s
clang-valgrind-intltest (regex)	5m 22s
clang-valgrind-intltest (format)	16m 3s
clang-valgrind-intltest (translit)	5m 45s
clang-valgrind-intltest (rbbi)	7m 46s
clang-valgrind-intltest (rbnf)	6m 12s
clang-valgrind-intltest (rbnfrt)	6m 32s
clang-valgrind-intltest (icuserv)	5m 25s
clang-valgrind-intltest (idna)	5m 14s
clang-valgrind-intltest (convert)	6m 46s
clang-valgrind-intltest (rbnfp)	5m 12s
clang-valgrind-intltest (csdet)	5m 30s
clang-valgrind-intltest (spoof)	5m 16s
clang-valgrind-intltest (bidi)	5m 29s
	1h 55m 17s

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