Merge pull request #74145 from tbkka/tbkka-remotemirror-mpe-fixes
Expand the work from #73491 to support more MPE layouts.
This is also switches the MPE layout code to exclusively use the new approach. The key observation: existing reflection metadata seems to already provide enough information in all cases, so we can abandon an earlier effort to add spare bitmask data.
There is some risk that the old code may have handled some enums that aren't (yet) fully supported by the new code. However, I did a bunch of experiments and found that our existing test cases have decent coverage of the major capabilities and I've also added a new test case specifically to exercise enum layouts being accessed resiliently. So if there are regressions from this change, they should be minor and easily fixed.
Resolves rdar://129281368
