Convert error to warning

Author: ergawy

flang/lib/Frontend/FrontendActions.cpp

@@ -366,11 +366,12 @@ bool CodeGenAction::beginSourceFileAction() {
   if (opts.doConcurrentMappingKind != DoConcurrentMappingKind::DCMK_None &&
       !isOpenMPEnabled) {
     unsigned diagID = ci.getDiagnostics().getCustomDiagID(
-        clang::DiagnosticsEngine::Error,
+        clang::DiagnosticsEngine::Warning,
         "lowering `do concurrent` loops to OpenMP is only supported if "
-        "OpenMP is enabled. Enable OpenMP using `-fopenmp`.");
+        "OpenMP is enabled. Enable OpenMP using `-fopenmp`. `do concurrent` "
+        "loops will be serialized.");
     ci.getDiagnostics().Report(diagID);
-    return false;
+    opts.doConcurrentMappingKind = DoConcurrentMappingKind::DCMK_None;
   }
 
   if (isOpenMPEnabled) {

flang/test/Transforms/DoConcurrent/command_line_options.f90

@@ -8,10 +8,10 @@
 ! BBC:      -fdo-concurrent-to-openmp=<string>
 ! BBC-SAME:   Try to map `do concurrent` loops to OpenMP [none|host|device] 
 
-! RUN: not %flang -fdo-concurrent-to-openmp=host %s 2>&1 \
+! RUN: %flang -fdo-concurrent-to-openmp=host %s 2>&1 \
 ! RUN: | FileCheck %s --check-prefix=OPT
 
-! OPT: error: lowering `do concurrent` loops to OpenMP is only supported if OpenMP is enabled.
+! OPT: warning: lowering `do concurrent` loops to OpenMP is only supported if OpenMP is enabled.
 ! OPT-SAME: Enable OpenMP using `-fopenmp`.
 
 program test_cli