ICU-22401 fix build on MSVC with cpp20 #298

Job	Run time
clang-valgrind-test	12m 51s
clang-valgrind-intltest (utility)	8m 17s
clang-valgrind-intltest (normalize)	7m 29s
clang-valgrind-intltest (collator)	7m 7s
clang-valgrind-intltest (regex)	8m 5s
clang-valgrind-intltest (format)	23m 4s
clang-valgrind-intltest (translit)	6m 57s
clang-valgrind-intltest (rbbi)	8m 31s
clang-valgrind-intltest (rbnf)	10m 2s
clang-valgrind-intltest (rbnfrt)	8m 50s
clang-valgrind-intltest (icuserv)	6m 59s
clang-valgrind-intltest (idna)	6m 6s
clang-valgrind-intltest (convert)	7m 54s
clang-valgrind-intltest (rbnfp)	7m 22s
clang-valgrind-intltest (csdet)	7m 0s
clang-valgrind-intltest (spoof)	5m 50s
clang-valgrind-intltest (bidi)	7m 55s
	2h 30m 19s

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