Validate GN substitutions in args and rsp files.

tools/gn/target_generator.cc

Index: tools/gn/target_generator.cc
diff --git a/tools/gn/target_generator.cc b/tools/gn/target_generator.cc
index 7fe64fd8b091ba5b1e3688b8c2aeffc52f45e060..ca193b646d9c1f11526a70bb4acd8de34bb72c7a 100644
--- a/tools/gn/target_generator.cc
+++ b/tools/gn/target_generator.cc
@@ -312,8 +312,9 @@ bool TargetGenerator::FillOutputs(bool allow_substitutions) {
   }
 
   // Check the substitutions used are valid for this purpose.
-  if (!EnsureValidSourcesSubstitutions(outputs.required_types(),
-                                       value->origin(), err_))
+  if (!EnsureValidSubstitutions(outputs.required_types(),
+                                &IsValidSourceSubstitution,
+                                value->origin(), err_))
     return false;
 
   // Validate that outputs are in the output dir.