[validation] impl v-0015: runtime array may only appear last

This CL adds checks to verify that runtime arrays only appear as the last element of a struct

Bug: tint:345
Change-Id: Ic2930aaf1e24e5c1d116add3a4a6dbdb9eaa02a7
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/33261
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Sarah Mashayekhi <sarahmashay@google.com>

src/validator/validator_impl.h

@@ -17,6 +17,7 @@
 
 #include <string>
 #include <unordered_map>
+#include <vector>
 
 #include "src/ast/assignment_statement.h"
 #include "src/ast/call_expression.h"
@@ -118,6 +119,12 @@ class ValidatorImpl {
   /// @returns true if the valdiation was successful
   bool ValidateEntryPoint(const ast::FunctionList& funcs);
 
+  /// Validates constructed types
+  /// @param constructed_types the types to check
+  /// @returns true if the valdiation was successful
+  bool ValidateConstructedTypes(
+      const std::vector<ast::type::Type*>& constructed_types);
+
  private:
   std::string error_;
   ScopeStack<ast::Variable*> variable_stack_;