Add support for code units > 0xFFFF in fill #7602

Job	Run time
mingw (mingw64)	4m 20s
build (windows-2019, Win32, v142, 17, Debug)	2m 15s
mingw (ucrt64)	3m 40s
build (windows-2019, Win32, v142, 17, Release)	2m 22s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	2m 14s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 15s
build (windows-2019, x64, v141, 14, Debug)	1m 42s
build (windows-2019, x64, v141, 14, Release)	1m 59s
build (windows-2019, x64, v141, 17, Debug)	1m 15s
build (windows-2019, x64, v141, 17, Release)	2m 23s
build (windows-2019, x64, v142, 17, Debug)	1m 28s
build (windows-2019, x64, v142, 17, Release)	2m 23s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 53s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 0s
build (windows-2019, x64, v142, 20, Debug)	2m 10s
build (windows-2019, x64, v142, 20, Release)	2m 27s
build (windows-2022, x64, v143, Debug, 20)	2m 19s
	39m 5s

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