add missing visitor for OmpMetadirectiveDirective

Author: kparzysz

flang/lib/Lower/OpenMP/OpenMP.cpp

@@ -413,6 +413,9 @@ extractOmpDirective(const parser::OpenMPConstruct &ompConstruct) {
                     [](const parser::OpenMPCancellationPointConstruct &c) {
                       return llvm::omp::OMPD_cancellation_point;
                     },
+                    [](const parser::OmpMetadirectiveDirective &c) {
+                      return llvm::omp::OMPD_metadirective;
+                    },
                     [](const parser::OpenMPDepobjConstruct &c) {
                       return llvm::omp::OMPD_depobj;
                     }},