add a higher-ranked match routine

Currently, when projecting out of a higher-ranked where-clause, we
instantiate all higher-ranked regions with lifetime variables. This is
unnecessary since the language rules ought to guarantee (modulo #32330)
that each of those higher-ranked regions is equated with some regions
from the input types. This routine figures out what those regions are
and just uses them. Also, since #32330 is not fully fixed, it detects
when we may have unconstrained variables and indicates that in its
return value.

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