[clang][OpenMP] Treat "workshare" as unknown OpenMP directive (#139793)

The "workshare" construct is only present in Fortran. The common OpenMP
code does treat it as any other directive, but in clang we need to
reject it, and do so gracefully before it encounters an internal
assertion.

Fixes #139424

Author: kparzysz

clang/lib/Parse/ParseOpenMP.cpp

@@ -2613,6 +2613,11 @@ StmtResult Parser::ParseOpenMPDeclarativeOrExecutableDirective(
     Diag(Tok, diag::err_omp_unknown_directive);
     return StmtError();
   }
+  if (DKind == OMPD_workshare) {
+    // "workshare" is an executable, Fortran-only directive. Treat it
+    // as unknown.
+    DKind = OMPD_unknown;
+  }
 
   StmtResult Directive = StmtError();
 

clang/test/OpenMP/openmp_workshare.c

@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -verify -fopenmp -ferror-limit 100 -o - %s
+
+// Workshare is a Fortran-only directive.
+
+void foo() {
+#pragma omp workshare // expected-error {{expected an OpenMP directive}}
+}
+