rustc_pass_by_value lint: add test on custom types

compiler/rustc_lint/src/pass_by_value.rs

@@ -11,6 +11,7 @@ declare_tool_lint! {
     /// The `rustc_pass_by_value` lint marks a type with `#[rustc_pass_by_value]` requiring it to always be passed by value.
     /// This is usually used for types that are thin wrappers around references, so there is no benefit to an extra
     /// layer of indirection. (Example: `Ty` which is a reference to a `TyS`)
+    /// This lint relies on `#[rustc_diagnostic_item]` being available for the target.
     pub rustc::PASS_BY_VALUE,
     Warn,
     "pass by reference of a type flagged as `#[rustc_pass_by_value]`",