Implement richer debug information for ProjectionElem

[klinvill]

Similar to what Oğuz did in rust-lang/rust#115534, indexed types should have debug methods that prints out more useful information like the name of the definition/variant/field.

As mentioned in rust-lang/rust#117517 (comment), this kind of debug printing is likely not possible for ProjectionElem until AdtDef is implemented in Stable MIR. Place::ty may also need to be implemented first to easily resolve the name of a variant/field. This is because it may be easiest to create a copy of the Place definition with the projection elements truncated to either the desired Downcast/Field projection or its parent, and then just call the Place::ty function to get the appropriate AdtDef.

[celinval]

I believe this issue is unblocked now sice we have implemented AdtDef and Place::ty(). @klinvill, any thoughts?

[klinvill]

Yeah I think it should be unblocked now. I unfortunately have a few other things on my plate to take care of first, but ideally can take a stab at this in the next couple of weeks.

[m-rph]

@rustbot claim

[m-rph]

This looks simple enough, unless I am missing something it's just a matter of adding all the variants of the enum, no?

[m-rph]

Okay, it seems that I am missing something, I can't see how the implementation of Place:ty can be used to retrieve the parent of an element. If all you have is local information, e.g. source relative index as in Downcast, how can Place::ty be used to retrieve the parent ADT?