Auto merge of #68057 - Aaron1011:fix/marker-trait-selection, r=matthewjasper

Auto merge of #68057 - Aaron1011:fix/marker-trait-selection, r=matthewjasper

Don't discard marker trait impls when inference variables are present

Fixes #61651

Previously, we would unconditionally discard impl candidates for marker
traits during trait selection. However, if the predicate had inference
variables, this could have the effect of constrainting inference
variables (due to a successful trait selection) when we would have
otherwise failed due to mutliple applicable impls,

This commit prevents marker trait impls from being discarded while the
obligation predicate has any inference variables, ensuring that
discarding impls will never cause us to incorrectly constraint inference
variables.