Remove the witness type from coroutine *args* (without actually removing the type)

[compiler-errors]

This does as much of #144157 as we can without having to break #143545 and/or introduce some better way of handling higher ranked assumptions.

Namely, it:

• Stalls coroutines based off of the coroutine type rather than the witness type.
• Reworks the dtorck constraint hack to not rely on the witness type.
• Removes the witness type from the args of the coroutine, eagerly creating the type for nested obligations when needed (auto/clone impls).

I'll experiment with actually removing the witness type in a follow-up.

r? lcnr

[rustbot]

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor