[clang] Resolve FIXME: Look at E, not M (PR #85541)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I assume the FIXME is referring to some deeper change here? If it were that easy, the FIXME wouldn't be there in the first place.

CC @zygoloid @rjmccall

https://github.com/llvm/llvm-project/pull/85541