split from #24425
Matching `tyGenericBody` performs a match on the last child of the
generic body, in this case the uninstantied `tyObject` type. If the
object contains no generic fields, this ends up being the same type as
all instantiated ones, but if it does, a new type is created. This fails
the `sameObjectTypes` check that subtype matching for object types uses.
To fix this, also consider that the pattern type could be the generic
uninstantiated object type of the matched type in subtype matching.