Auto merge of #103390 - compiler-errors:metadata-mod-regions, r=eholk

Check fat pointer metadata compatibility modulo regions

Regions don't really mean anything anyways during hir typeck.

If this `erase_regions` makes anyone nervous, it's probably equally valid to just equate the types using a type relation, but regardless we should _not_ be using strict type equality while region variables are present.

Fixes #103384

Author: bors