Auto merge of #37541 - nikomatsakis:issue-37291, r=brson

Use impl obligations as initial environment for specialization

This corrects a small regression in specialization that crept in, I think as part of the refactoring to introduce arenas. I also made an experiment (in the last commit) to cleanup the code to be more aggressive about normalization. As the commit log notes, I am not 100% sure that this is correct, but it feels safer, and I think that at worst it yields *more* ICEs (as opposed to admitting faulty code). I'll schedule a crater run to check beyond the testbase.

Fixes #37291.

r? @aturon