Add support for _BitInt on clang #6611

Job	Run time
build (g++-4.8, Debug, 11)	1m 20s
build (g++-4.8, Release, 11)	1m 56s
build (g++-10, Debug, 11)	1m 22s
build (g++-10, Release, 11)	2m 4s
build (clang++-9, Debug, 11)	1m 23s
build (clang++-9, Release, 11)	2m 19s
build (g++-8, Debug, 14, sudo apt install g++-8)	1m 38s
build (g++-8, Debug, 17, sudo apt install g++-8)	1m 32s
build (g++-9, Debug, 17)	1m 44s
build (g++-10, Debug, 17)	1m 34s
build (g++-11, Debug, 20, sudo apt install g++-11)	2m 10s
build (clang++-8, Debug, 17, -stdlib=libc++, sudo apt install clang-8 libc++-8-dev libc++abi-8-dev)	1m 35s
build (clang++-9, Debug, 17, -DFMT_FUZZ=ON -DFMT_FUZZ_LINKMAIN=ON, sudo apt install clang-9)	1m 41s
build (clang++-11, Debug, 20)	1m 52s
build (clang++-11, Debug, 20, -stdlib=libc++, sudo apt install libc++-11-dev libc++abi-11-dev)	1m 40s
	25m 50s

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