Auto merge of #44269 - nikomatsakis:issue-43613-limit-cache-obligations, r=arielb1

limit and clear cache obligations opportunistically

Keeping **all** the obligations for every projection is wasteful of
memory and compilation time. We only really care about those
subobligations that may inform the result of the projection (i.e., may
help to resolve any inference variables that appear within).
Therefore, we can clear the subobligations from the cache that don't
potentially affect the result of the projection. On every cache hit,
we also take the opportunity to check if the type variables have been
resolved *yet* and, if so, clear out the pending obligations.

Fixes #43613.

r? @arielb1

NB -- not sure how to test for this. Probably we should add the #43613 test case to perf.

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