[flang][driver] add warning when using openmp

Author: everythingfunctional

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -147,6 +147,9 @@ def warn_drv_unsupported_option_for_processor : Warning<
 def warn_drv_unsupported_openmp_library : Warning<
   "the library '%0=%1' is not supported, OpenMP will not be enabled">,
   InGroup<OptionIgnored>;
+def warn_openmp_experimental : Warning<
+  "OpenMP support in flang is still experimental">,
+  InGroup<ExperimentalOption>;
 
 def err_drv_invalid_thread_model_for_target : Error<
   "invalid thread model '%0' in '%1' for this target">;

clang/include/clang/Basic/DiagnosticGroups.td

@@ -1583,3 +1583,7 @@ def ExtractAPIMisuse : DiagGroup<"extractapi-misuse">;
 // Warnings about using the non-standard extension having an explicit specialization
 // with a storage class specifier.
 def ExplicitSpecializationStorageClass : DiagGroup<"explicit-specialization-storage-class">;
+
+// A warning for options that enable a feature that is not yet complete
+def ExperimentalOption : DiagGroup<"experimental-option">;
+

clang/lib/Driver/ToolChains/Flang.cpp

@@ -787,6 +787,9 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,
 
       if (Args.hasArg(options::OPT_fopenmp_force_usm))
         CmdArgs.push_back("-fopenmp-force-usm");
+      // TODO: OpenMP support isn't "done" yet, so for now we warn that it
+      // is experimental.
+      D.Diag(diag::warn_openmp_experimental);
 
       // FIXME: Clang supports a whole bunch more flags here.
       break;