ICU-22401 fix build on MSVC with cpp20 #346

Job	Run time
clang-valgrind-test	13m 45s
clang-valgrind-intltest (utility)	12m 21s
clang-valgrind-intltest (normalize)	10m 11s
clang-valgrind-intltest (collator)	6m 15s
clang-valgrind-intltest (regex)	6m 0s
clang-valgrind-intltest (format)	25m 15s
clang-valgrind-intltest (translit)	9m 0s
clang-valgrind-intltest (rbbi)	10m 1s
clang-valgrind-intltest (rbnf)	11m 5s
clang-valgrind-intltest (rbnfrt)	7m 20s
clang-valgrind-intltest (icuserv)	6m 37s
clang-valgrind-intltest (idna)	6m 44s
clang-valgrind-intltest (convert)	8m 13s
clang-valgrind-intltest (rbnfp)	8m 26s
clang-valgrind-intltest (csdet)	6m 3s
clang-valgrind-intltest (spoof)	6m 18s
clang-valgrind-intltest (bidi)	6m 55s
	2h 40m 29s

Provide feedback