Validator: Change the error message for v-0031

It previously said:

"a struct containing a runtime-sized array must be in the 'storage' storage class"

This had be looking at the variable's storage class, when the error was actually trying to tell me I needed `[[block]]` on the struct declaration.

Change-Id: I7a23a0c0c35508bdac20c808d2635592638dfa77
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/40602
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>

src/validator/validator_impl.cc

diff --git a/src/validator/validator_impl.cc b/src/validator/validator_impl.cc
index 16da3e3..ecad8ef 100644
--- a/src/validator/validator_impl.cc
+++ b/src/validator/validator_impl.cc
@@ -318,9 +318,9 @@
               return false;
             }
             if (!st->IsBlockDecorated()) {
-              add_error(member->source(), "v-0031",
+              add_error(member->source(), "v-0015",
                         "a struct containing a runtime-sized array "
-                        "must be in the 'storage' storage class: '" +
+                        "requires the [[block]] attribute: '" +
                             program_->Symbols().NameFor(st->symbol()) + "'");
               return false;
             }

src/validator/validator_type_test.cc

diff --git a/src/validator/validator_type_test.cc b/src/validator/validator_type_test.cc
index 306182a..5f136ea 100644
--- a/src/validator/validator_type_test.cc
+++ b/src/validator/validator_type_test.cc
@@ -79,8 +79,8 @@
 
   EXPECT_FALSE(v.ValidateConstructedTypes(program->AST().ConstructedTypes()));
   EXPECT_EQ(v.error(),
-            "v-0031: a struct containing a runtime-sized array must be "
-            "in the 'storage' storage class: 'Foo'");
+            "v-0015: a struct containing a runtime-sized array requires the "
+            "[[block]] attribute: 'Foo'");
 }
 
 TEST_F(ValidatorTypeTest, RuntimeArrayIsNotLast_Fail) {