Auto merge of #125610 - oli-obk:define_opaque_types14, r=compiler-errors

Allow constraining opaque types during various unsizing casts

allows unsizing of tuples, arrays and Adts to constraint opaque types in their generic parameters to concrete types on either side of the unsizing cast.

Also allows constraining opaque types during trait object casts that only differ in auto traits or lifetimes.

cc #116652

Author: bors