Passing on a request from the developers of the FEX x86 emulator:

While building mixed-architecture software, they've been frustrated by
the fact that files in /usr/lib/llvm-19 are not multi-arch
co-installable, so on Debian/Ubuntu they can't build amd64 and i386 code
that uses libLLVM (such as 64- and 32-bit versions of Mesa) without
entering separate containers/chroots.

This affects llvm-{19,20}-dev and perhaps others.

I'm told that on other distributions like the Red Hat and Arch families,
the equivalent of our /usr/lib/llvm-19 is something like
/usr/lib{64,}/llvm (Red Hat) or /usr/lib{,32}/llvm (Arch). This suggests
that on Debian, it should be possible to use our equivalent of lib64,
/usr/lib/${DEB_HOST_MULTIARCH}/llvm-{19,20,...}, for a similar level of
co-installability.

Thanks,
    smcv