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Fix `Wimplicit-fallthrough` warnings (#713) #4060

Job	Run time
Alpine Debug x86_64 (gcc)	6m 21s
macOS Debug	11m 59s
Alpine Debug x86 (gcc)	6m 59s
macOS Release	8m 28s
Alpine Release x86_64 (gcc)	10m 5s
Alpine Release x86 (gcc)	9m 3s
Ubuntu Debug x64 (gcc)	6m 40s
Ubuntu Debug x64 (clang)	6m 58s
Ubuntu Release x64 (gcc)	6m 24s
Windows Debug x64 (msvc)	7m 44s
Ubuntu Release x64 (clang)	10m 6s
Windows Debug x86 (msvc)	8m 9s
Windows Mixed x64 (msvc)	6m 44s
Windows Mixed x86 (msvc)	6m 16s
Windows Release x64 (msvc)	12m 17s
Windows Release x86 (msvc)	10m 33s
Windows Release Master Gold x64 (msvc)	10m 31s
Windows Release Master Gold x86 (msvc)	9m 30s
	2h 34m 47s

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