Don't repeat AssertParamIs{Clone,Eq} assertions.

It's common to see repeated assertions like this in derived `clone` and
`eq` methods:
```
let _: ::core::clone::AssertParamIsClone<u32>;
let _: ::core::clone::AssertParamIsClone<u32>;
```
This commit avoids them.

compiler/rustc_ast/src/ast.rs

@@ -2036,6 +2036,14 @@ impl TyKind {
     pub fn is_unit(&self) -> bool {
         matches!(self, TyKind::Tup(tys) if tys.is_empty())
     }
+
+    pub fn is_simple_path(&self) -> Option<Symbol> {
+        if let TyKind::Path(None, Path { segments, .. }) = &self && segments.len() == 1 {
+            Some(segments[0].ident.name)
+        } else {
+            None
+        }
+    }
 }
 
 /// Syntax used to declare a trait object.