[Tests] NFC: Add a FIXME about use of <<error type>> in a diagnostic

(cherry picked from commit 4c38d72)

Author: xedin

validation-test/Sema/type_checker_crashers_fixed/rdar116122902.swift

@@ -14,6 +14,7 @@ struct Test {
 
   mutating func test<T>(v: Value<T>) {
     _ = {
+      // FIXME(diagnostics): We need to figure out how to avoid mentioning <<error type>> in the second diagnostic
       self.tuple = (v, 42)
       // expected-error@-1 {{cannot assign value of type '(Value<T>, Int)' to type '(value: any AnyValue, id: Int)'}}
       // expected-error@-2 {{generic struct 'Value' requires the types 'T' and '<<error type>>' be equivalent}}