Stop using inherent impl to generate auto/blanket impl

[notriddle]

rust/src/librustdoc/passes/collect_trait_impls.rs

Lines 84 to 89 in 4c5f6e6

	if did.is_local() {
	for def_id in prim.impls(cx.tcx) {
	let impls = get_auto_trait_and_blanket_impls(cx, def_id);
	new_items_external.extend(impls.filter(|i| cx.inlined.insert(i.item_id)));
	}
	}

This tactic of using inherent impl blocks for getting
auto traits and blanket impls is a hack. What we really
want is to check if [T] impls Send, which has
nothing to do with the inherent impl.

Rustdoc currently uses these impl block as a source of
the Ty, as well as the ParamEnv, SubstsRef, and
Generics. To avoid relying on the impl block, these
things would need to be created from wholecloth, in a
form that is valid for use in type inference.