Fix precise field capture of univariant enums
When constructing a MIR from a THIR field expression, introduce an
additional downcast projection before accessing a field of an enum.
When rebasing a place builder on top of a captured place, account for
the fact that a single HIR enum field projection corresponds to two MIR
projection elements: a downcast element and a field element.
Fixes#95271.
Fixes#96299.
Fixes#96512.
Fixes#97378.
r? ``@nikomatsakis`` ``@arora-aman``
fd76e0eee0