Use PostBorrowckAnalysis in check_coroutine_obligations

[compiler-errors]

This currently errors with:

error: concrete type differs from previous defining opaque type use
  --> tests/ui/coroutine/issue-52304.rs:10:21
   |
10 | pub fn example() -> impl Coroutine {
   |                     ^^^^^^^^^^^^^^ expected `{example::{closure#0} upvar_tys=() resume_ty=() yield_ty=&'{erased} i32 return_ty=() witness={example::{closure#0}}}`, got `{example::{closure#0} upvar_tys=() resume_ty=() yield_ty=&'static i32 return_ty=() witness={example::{closure#0}}}`
   |
   = note: previous use here

This is because we end up redefining the opaque in check_coroutine_obligations but with the yield_ty = &'erased i32 from hir typeck, which causes the equality check for opaques to fail.

The coroutine obligtions in question (when -Znext-solver is enabled) are:

Binder { value: TraitPredicate(<Opaque(DefId(0:5 ~ issue_52304[4c6d]::example::{opaque#0}), []) as std::marker::Sized>, polarity:Positive), bound_vars: [] }

Binder { value: AliasRelate(Term::Ty(Alias(Opaque, AliasTy { args: [], def_id: DefId(0:5 ~ issue_52304[4c6d]::example::{opaque#0}), .. })), Equate, Term::Ty(Coroutine(DefId(0:6 ~ issue_52304[4c6d]::example::{closure#0}), [(), (), &'{erased} i32, (), CoroutineWitness(DefId(0:6 ~ issue_52304[4c6d]::example::{closure#0}), []), ()]))), bound_vars: [] }

Binder { value: AliasRelate(Term::Ty(Coroutine(DefId(0:6 ~ issue_52304[4c6d]::example::{closure#0}), [(), (), &'{erased} i32, (), CoroutineWitness(DefId(0:6 ~ issue_52304[4c6d]::example::{closure#0}), []), ()])), Subtype, Term::Ty(Alias(Opaque, AliasTy { args: [], def_id: DefId(0:5 ~ issue_52304[4c6d]::example::{opaque#0}), .. }))), bound_vars: [] }

Ignoring the fact that we end up stalling some really dumb obligations here (lol), I think it makes more sense for us to be using post borrowck analysis for this check anyways.

r? lcnr