This is surprisingly complicated!
Alpine appears to use gcc to build by default: https://git.alpinelinux.org/abuild/tree/abuild.in#n28
Three Linux distros build with LLVM: Android and ChromeOS (both built by Google) and OpenMandriva (experimental clang support for kernel builds only) https://www.kernel.org/doc/html/latest/kbuild/llvm.html
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