[Diagnostics] Stage in the new error message for existential types that

must be marked with 'any' as a warning.

Author: hborla

lib/Sema/TypeCheckType.cpp

@@ -4085,7 +4085,8 @@ class ExistentialTypeVisitor
       if (proto->existentialRequiresAny()) {
         Ctx.Diags.diagnose(comp->getNameLoc(),
                            diag::existential_requires_any,
-                           proto->getName());
+                           proto->getName())
+            .limitBehavior(DiagnosticBehavior::Warning);
       }
     } else if (auto *alias = dyn_cast_or_null<TypeAliasDecl>(comp->getBoundDecl())) {
       auto type = Type(alias->getDeclaredInterfaceType()->getDesugaredType());
@@ -4101,7 +4102,8 @@ class ExistentialTypeVisitor
 
             Ctx.Diags.diagnose(comp->getNameLoc(),
                                diag::existential_requires_any,
-                               protoDecl->getName());
+                               protoDecl->getName())
+                .limitBehavior(DiagnosticBehavior::Warning);
           }
         }
         return false;