Add `BOOST_FALLTHROUGH` for MSVC #404

Job	Run time
emscripten_wasm (emcc)	1m 48s
non_intel_ubuntu_20_04_clang (aarch64)	17m 44s
macos_11 (clang)	1s
non_intel_ubuntu_20_04_clang (ppc64le)	17m 47s
macos_12 (clang)	1m 57s
macos_11 (gcc-11)	1s
non_intel_ubuntu_20_04_gcc (aarch64)	17m 15s
macos_11 (gcc-10)	1s
non_intel_ubuntu_20_04_gcc (s390x)	16m 57s
ubuntu-cuda (11)	4m 25s
ubuntu-focal (g++-9)	2m 5s
non_intel_ubuntu_20_04_gcc (ppc64le)	17m 9s
ubuntu-cuda (14)	3m 56s
ubuntu-focal (g++-10)	2m 2s
ubuntu-focal-clang (clang++-11)	1m 46s
ubuntu-cuda (17)	3m 55s
ubuntu-focal (clang++-9)	1m 58s
ubuntu-focal-clang (clang++-12)	1m 57s
ubuntu-jammy (g++-11)	2m 27s
ubuntu-focal (clang++-10)	2m 7s
ubuntu-jammy (g++-12)	2m 10s
ubuntu-jammy-clang (clang++-15)	1m 49s
windows_clang_msvc_14_3 (clang-msvc-14.3)	2m 25s
ubuntu-jammy (g++-13)	1m 55s
windows_msvc_14_0 (msvc-14.0)	2m 8s
windows_msvc_14_2 (msvc-14.2)	2m 23s
windows_msvc_14_3 (msvc-14.3)	2m 19s
	2h 12m 27s

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