Ignore but do not assume region obligations from unifying headers in negative coherence

[compiler-errors]

Partly addresses a FIXME that was added in #112875. Just as we can throw away the nested trait/projection obligations from unifying two impl headers, we can also just throw away the region obligations too.

I removed part of the FIXME that was incorrect, namely:

Given that the only region constraints we get are involving inference regions in the root, it shouldn't matter, but still sus.

This is not true when unifying fn(A) and for<'b> fn(&'b B) which ends up with placeholder region outlives from non-root universes. I'm pretty sure this is okay, though it would be nice if we were to use them as assumptions. See the explicit revision of the test I committed, which still fails.

Fixes #117986

r? lcnr, feel free to reassign tho.