Remove `DefiningAnchor::Bubble` from opaque wf check
Set the defining anchor to `DefiningAnchor::Bind(parent_def_id)` where `parent_def_id` is the first parent def-id that isn't an opaque.
This "fixes" some of the nested-return-type wf tests. If we *do* want these to be hard-errors for TAITs, we should probably make those error separately from this check (i.e. via some check like the code in the `OPAQUE_HIDDEN_INFERRED_BOUND` lint). The fact that some of these tests fail but not all of them seems kinda coincidental.
r? oli-obk
b0572f1a12