Don't ICE on errors in function returning impl trait

Instead, report the error.

This emits the errors on-demand, without special-casing `impl Trait`, so
it should catch all ICEs of this kind, including ones that haven't been
found yet.

Since the error is emitted during type-checking there is less info about
the error; see comments in the code for details.

- Add test case for -> impl Trait
- Add test for impl trait with alias
- Move EmitIgnoredResolutionErrors to rustdoc

This makes `fn typeck_item_bodies` public, which is not desired behavior.
That change should be removed once
https://github.com/rust-lang/rust/pull/74070 is merged.

- Don't visit nested closures twice

src/test/rustdoc/impl-trait-alias.rs

@@ -0,0 +1,14 @@
+#![feature(type_alias_impl_trait)]
+
+trait MyTrait {}
+impl MyTrait for i32 {}
+
+// @has impl_trait_alias/type.Foo.html 'Foo'
+/// debug type
+pub type Foo = impl MyTrait;
+
+// @has impl_trait_alias/fn.foo.html 'foo'
+/// debug function
+pub fn foo() -> Foo {
+    1
+}