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/impl-keyword.rs

@@ -0,0 +1,6 @@
+pub trait ValidTrait {}
+/// This returns impl trait
+pub fn g() -> impl ValidTrait {
+    error::_in::impl_trait()
+    //~^ ERROR failed to resolve
+}