trait_sel: resolve vars in host effects

In the standard library, the `Extend` impl for `Iterator` (specialised
with `TrustedLen`) has a parameter which is constrained by a projection
predicate. This projection predicate provides a value for an inference
variable but host effect evaluation wasn't resolving variables first.

Adding the extra resolve can the number of errors in some tests when they
gain host effect predicates, but this is not unexpected as calls to
`resolve_vars_if_possible` can cause more error tainting to happen.

Co-authored-by: Boxy <rust@boxyuwu.dev>

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