Add allow-by-default lint for unit bindings

This lint is not triggered if any of the following conditions are met:

- The user explicitly annotates the binding with the `()` type.
- The binding is from a macro expansion.
- The user explicitly wrote `let () = init;`
- The user explicitly wrote `let pat = ();`. This is allowed for local
  lifetimes.

compiler/rustc_lint/src/lints.rs

@@ -1845,3 +1845,10 @@ impl<'a> DecorateLint<'a, ()> for AsyncFnInTraitDiag {
         fluent::lint_async_fn_in_trait
     }
 }
+
+#[derive(LintDiagnostic)]
+#[diag(lint_unit_bindings)]
+pub struct UnitBindingsDiag {
+    #[label]
+    pub label: Span,
+}