[flang][openacc][NFC] Update _OPENACC definition to 202011

Update _OPENACC definition to be consistent with the flang-new
driver. Currently set to 202011 which is OpenACC 3.1 specification and
is the current parser/semantic status.

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D150400

flang/tools/bbc/bbc.cpp

@@ -375,7 +375,7 @@ int main(int argc, char **argv) {
   // enable parsing of OpenACC
   if (enableOpenACC) {
     options.features.Enable(Fortran::common::LanguageFeature::OpenACC);
-    options.predefinitions.emplace_back("_OPENACC", "201911");
+    options.predefinitions.emplace_back("_OPENACC", "202011");
   }
 
   Fortran::common::IntrinsicTypeDefaultKinds defaultKinds;