In #31717 we rebased our LLVM fork over the 3.8 release branch, and it was
thought that this fixed#31702. The testing, however, must have been erroneous,
as it unfortunately didn't fix the issue! Our MUSL nightly builders are failing
from the same assertion reported in the issue, so we at least know the test case
is a reproduction!
I believe the failure is only happening on the MUSL nightly builders because
none of the auto builders have LLVM assertions enabled, and the Linux nightly
builder *does* have assertions enabled for the binaries we generate but the
distcheck run doesn't test a compiler with LLVM assertions enabled.
This commit rebases our LLVM submodule on the most recent tip of the
`release_38` branch of LLVM. There's been a few fixes and this notably fixes the
assertion error in #31702.
