[LLVM][OpenMP] Add "nowait" clause as valid for "workshare" (#88426)

Add the "nowait" clause to the list of allowed clauses for the "workshare"
directive. This will make it consistent with other directives (which are
shared between C/C++ and Fortran).

The parser will still reject "nowait" on "!$omp workshare", so this has no
effect on accepting/rejecting Fortran source code.

flang/lib/Semantics/check-omp-structure.h

@@ -42,6 +42,7 @@ static const OmpDirectiveSet noWaitClauseNotAllowedSet{
     Directive::OMPD_do_simd,
     Directive::OMPD_sections,
     Directive::OMPD_single,
+    Directive::OMPD_workshare,
 };
 } // namespace omp
 } // namespace llvm

flang/test/Semantics/OpenMP/clause-validity01.f90

@@ -342,6 +342,9 @@
   a = 1.0
   !ERROR: COPYPRIVATE clause is not allowed on the END WORKSHARE directive
   !$omp end workshare nowait copyprivate(a)
+  !$omp workshare nowait
+  !ERROR: NOWAIT clause is not allowed on the WORKSHARE directive, use it on OMP END WORKSHARE directive
+  !$omp end workshare
   !$omp end parallel
 
 ! 2.8.1 simd-clause -> safelen-clause |

llvm/include/llvm/Frontend/OpenMP/OMP.td

@@ -2115,6 +2115,9 @@ def OMP_scope : Directive<"scope"> {
   let association = AS_Block;
 }
 def OMP_Workshare : Directive<"workshare"> {
+  let allowedOnceClauses = [
+    VersionedClause<OMPC_NoWait>
+  ];
   let association = AS_Block;
 }
 def OMP_ParallelWorkshare : Directive<"parallel workshare"> {