Auto merge of #44743 - arielb1:size-rollback, r=eddyb

typeck::check::coercion - roll back failed unsizing type vars

This wraps unsizing coercions within an additional level of
`commit_if_ok`, which rolls back type variables if the unsizing coercion
fails. This prevents a large amount of type-variables from accumulating
while type-checking a large function, e.g. shaving 2GB off one of the
4GB peaks in #36799.

This is a performance-sensitive PR so please don't roll it up.

r? @eddyb
cc @nikomatsakis

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