Don't try to add nested predicate to Rustdoc auto-trait ParamEnv

Fixes #80233

We already have logic in `evaluate_predicates` that tries to add
unimplemented predicates to our `ParamEnv`. Trying to add a predicate
that already holds can lead to errors later on, since projection
will prefer trait candidates from the `ParamEnv` to predicates from an
impl.

src/test/rustdoc/issue-80233-normalize-auto-trait.rs

@@ -0,0 +1,37 @@
+// Regression test for issue #80233
+// Tests that we don't ICE when processing auto traits
+
+#![crate_type = "lib"]
+pub trait Trait1 {}
+
+pub trait Trait2 {
+    type Type2;
+}
+
+pub trait Trait3 {
+    type Type3;
+}
+
+impl Trait2 for Struct1 {
+    type Type2 = Struct1;
+}
+
+impl<I: Trait2> Trait2 for Vec<I> {
+    type Type2 = Vec<I::Type2>;
+}
+
+impl<T: Trait1> Trait3 for T {
+    type Type3 = Struct1;
+}
+
+impl<T: Trait3> Trait3 for Vec<T> {
+    type Type3 = Vec<T::Type3>;
+}
+
+pub struct Struct1 {}
+
+// @has issue_80233_normalize_auto_trait/struct.Question.html
+// @has - '//code' 'impl<T> Send for Question<T>'
+pub struct Question<T: Trait1> {
+    pub ins: <<Vec<T> as Trait3>::Type3 as Trait2>::Type2,
+}