Auto merge of #64584 - nikomatsakis:issue-64477-generator-capture-types, r=eddyb

Auto merge of #64584 - nikomatsakis:issue-64477-generator-capture-types, r=eddyb

record fewer adjustment types in generator witnesses, avoid spurious drops in MIR construction

Don't record all intermediate adjustment types -- That's way more than is needed, and winds up recording types that will never appear in MIR.

Note: I'm like 90% sure that this logic is correct, but this stuff is subtle and can be hard to keep straight.  However, the risk of this PR is fairly low -- if we miss types here, I believe the most common outcome is an ICE.

This fixes the original issue cited by #64477, but I'm leaving the issue open for now since there may be other cases we can detect and improve in a targeted way.

r? @Zoxc