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_mir/src/borrow_check/region_infer/mod.rs

@@ -1241,7 +1241,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// it. However, it works pretty well in practice. In particular,
     /// this is needed to deal with projection outlives bounds like
     ///
-    /// ```ignore (internal compiler representation so lifetime syntax is invalid)
+    /// ```text (internal compiler representation so lifetime syntax is invalid)
     /// <T as Foo<'0>>::Item: '1
     /// ```
     ///