[gn] Tag unneeded variable.

When only building targets without assembly parsers (e.g. `llvm_targets_to_build
= ["NVPTX"]`), `all_targets` is empty and causes GN to warn about an
assigned-but-unused variable.

Differential Revision: https://reviews.llvm.org/D31727

llvm-svn: 365897

llvm/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn

@@ -383,6 +383,9 @@ template("write_target_def_file") {
     foreach(target, all_targets) {
       value = "$value${invoker.value}($target)\n"
     }
+    if (all_targets == []) {
+      not_needed(invoker, [ "value" ])
+    }
     values = [ "${invoker.key}=$value" ]
   }
 }