mir: add `used_generic_parameters_needs_subst`
Fixes#74636.
This PR adds a `used_generic_parameters_needs_subst` helper function which checks whether a type needs substitution, but only for parameters that the `unused_generic_params` query considers used. This is used in the MIR interpreter to make the check for some pointer casts and for reflection intrinsics more precise.
I've opened this as a draft PR because this might not be the approach we want to fix this issue and we have to decide what to do about the reflection case.
r? @eddyb
cc @lcnr @wesleywiser
22e6099330