rustc_typeck: turn `where Type:,` into a WF(Type) predicate, instead of ignoring it.
Fixes#53696, assuming crater doesn't catch anyone using this syntax already.
Allowing an empty list of bounds in the grammar was done for the benefit of macro authors, most of which would *probably* be using it for bounds on type parameters, which are always WF.
r? @nikomatsakis cc @petrochenkov