New rustdoc lint to respect -Dwarnings correctly

This adds a new lint to `rustc` that is used in rustdoc when a code
block is empty or cannot be parsed as valid Rust code.

Previously this was unconditionally a warning. As such some
documentation comments were (unknowingly) abusing this to pass despite
the `-Dwarnings` used when compiling `rustc`, this should not be the
case anymore.

compiler/rustc_trait_selection/src/opaque_types.rs

@@ -46,6 +46,7 @@ pub struct OpaqueTypeDecl<'tcx> {
     /// type Foo = impl Baz;
     /// fn bar() -> Foo {
     /// //          ^^^ This is the span we are looking for!
+    /// }
     /// ```
     ///
     /// In cases where the fn returns `(impl Trait, impl Trait)` or