Add support for code units > 0xFFFF in fill #7606

Job	Run time
build (windows-2019, Win32, v142, 17, Debug)	1m 6s
build (windows-2019, Win32, v142, 17, Release)	2m 24s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 58s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 12s
build (windows-2019, x64, v141, 14, Debug)	2m 3s
build (windows-2019, x64, v141, 14, Release)	2m 38s
build (windows-2019, x64, v141, 17, Debug)	1m 20s
build (windows-2019, x64, v141, 17, Release)	2m 34s
mingw (mingw64)	3m 31s
build (windows-2019, x64, v142, 17, Debug)	1m 32s
mingw (ucrt64)	3m 58s
build (windows-2019, x64, v142, 17, Release)	2m 23s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 29s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 24s
build (windows-2019, x64, v142, 20, Debug)	1m 29s
build (windows-2019, x64, v142, 20, Release)	2m 44s
build (windows-2022, x64, v143, Debug, 20)	2m 20s
	38m 5s

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