Recurse into function bodies, but don't typeck closures

Previously, rustdoc would issue a delay_span_bug ICE on the following code:

```rust
pub fn a() -> impl Fn() -> u32 {
    || content::doesnt::matter()
}
```

This wasn't picked up earlier because having `type Alias = impl Trait;`
in the same module caused _all closures_ to be typechecked, even if they
wouldn't normally. Additionally, if _any_ error was emitted, no
delay_span_bug would be emitted. So as part of this commit all of the
tests were separated out into different files.

src/test/rustdoc-ui/error-in-impl-trait/trait-alias.rs

@@ -0,0 +1,10 @@
+#![feature(type_alias_impl_trait)]
+
+pub trait ValidTrait {}
+type ImplTrait = impl ValidTrait;
+
+/// This returns impl trait, but using a type alias
+pub fn h() -> ImplTrait {
+    error::_in::impl_trait::alias()
+    //~^ ERROR failed to resolve
+}