Your issue (libllvm12:amd64 and libllvm12:i386 not co-installable) is most probably due to the i386 build of libllvm12 1:12.0.1-10 failing, leading to its absence from Debian Sid repositories. You can see more details about this build failure here: https://bugs.debian.org/996796
I expect libllvm12:i386 1:12.0.1-10 to be available as soon as #996796 is fixed. In the meantime, if you need this package in both architecture you would need to stay on 1:12.0.1-9 even for the amd64 build. You can download this package from snapshot.debian.org: https://snapshot.debian.org/package/llvm-toolchain-12/1%3A12.0.1-9/#libllvm12_1:3a:12.0.1-9
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