To answer your incidental question, Raspian is maintained by Raspberry Pi folks, not Debian.
But I haven't yet found a similar repository on the Raspbian side. I guess I'd expect to find it within their GitHub org, but my searching didn't reveal it.
[1] https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/comm...
Anyway, I suppose this all means Raspberry Pi OS/Raspbian are able to patch this, without requiring it to first be fixed in Debian or Clang.
[1] https://raspberrypi.stackexchange.com/questions/1179/does-ra...
Uncovering this slight difference really makes me long for something like the Debian GitLab instance (or really any kind of public change tracking) to document a bug or suggest a change.
Agreed, sure looks like it's Raspbian's build configuration to fix.