Remove define_debug_via_print for ExistentialProjection, use regular structural debug impl

[compiler-errors]

The pretty print impl for ExistentialProjection always prints AssocItem = Ty:

rust/compiler/rustc_middle/src/ty/print/pretty.rs

Lines 3293 to 3299 in 6e83046

	ty::ExistentialProjection<'tcx> {
	let name = cx.tcx().associated_item(self.def_id).name;
	// The args don't contain the self ty (as it has been erased) but the corresp.
	// generics do as the trait always has a self ty param. We need to offset.
	let args = &self.args[cx.tcx().generics_of(self.def_id).parent_count - 1..];
	p!(path_generic_args(|cx| write!(cx, "{name}"), args), " = ", print(self.term))
	}

We can't change this, b/c it's used for both pretty printing dyn types and for legacy symbol mangling.

Unfortunately, we also use this printing procedure for Debug impls. That means that it leaves out the trait name and trait args when debug printing an ExistentialProjection (or an ExistentialPredicate which has a variant for ExistentialProjection). This leads to awkward situations, like the two seemingly identical existential projection predicates present in a dyn Trait type using the definition below:

trait Super { type Assoc; }

trait Foo: Super<A, Assoc = i32> + Super<B, Assoc = i32> {}

Namely, they both just render as Projection(Assoc = i32)! This makes debugging dyn Trait type system bugs really hard, so let's use the regular debug impl for ExistentialProjection.

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[lcnr]

@bors r+ rollup

[bors]

📌 Commit c289b94 has been approved by lcnr

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[bors]

☔ The latest upstream changes (presumably #139781) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

@bors r=lcnr

[bors]

📌 Commit 1b9ae74 has been approved by lcnr

It is now in the queue for this repository.

[compiler-errors]

@bors r-

[compiler-errors]

@bors r=lcnr

[bors]

📌 Commit 83ea345 has been approved by lcnr

It is now in the queue for this repository.