rustdoc-search: show impl Trait inline when unhighlighted

While normal generics can be skipped in this case, no-names need
something to show here.

Before: `TyCtxt, , Symbol -> bool`

After: `TyCtxt, Into<DefId>, Symbol -> bool`

tests/rustdoc-js/impl-trait-inlining.rs

@@ -0,0 +1,11 @@
+#![crate_name="foo"]
+
+pub struct TyCtxt;
+pub struct DefId;
+pub struct Symbol;
+
+impl TyCtxt {
+    pub fn has_attr(self, _did: impl Into<DefId>, _attr: Symbol) -> bool {
+        unimplemented!();
+    }
+}