[OPENMP50]Add support for nested atomic and simd constructs in

simd-based directives.

According to OpenMP 5.0 standard, ordered simd, atomic and simd
directives are allowed as nested directives in the simd-based
directives.

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -9312,7 +9312,7 @@ def err_omp_prohibited_region : Error<
   "; perhaps you forget to enclose 'omp %3' directive into a target region?|"
   "; perhaps you forget to enclose 'omp %3' directive into a teams region?}2">;
 def err_omp_prohibited_region_simd : Error<
-  "OpenMP constructs may not be nested inside a simd region">;
+  "OpenMP constructs may not be nested inside a simd region%select{| except for ordered simd, simd or atomic directive}0">;
 def err_omp_prohibited_region_atomic : Error<
   "OpenMP constructs may not be nested inside an atomic region">;
 def err_omp_prohibited_region_critical_same_name : Error<

clang/lib/Sema/SemaOpenMP.cpp

@@ -3799,7 +3799,10 @@ static bool checkNestingOfRegions(Sema &SemaRef, const DSAStackTy *Stack,
       ShouldBeInTargetRegion,
       ShouldBeInTeamsRegion
     } Recommend = NoRecommend;
-    if (isOpenMPSimdDirective(ParentRegion) && CurrentRegion != OMPD_ordered) {
+    if (isOpenMPSimdDirective(ParentRegion) &&
+        ((SemaRef.LangOpts.OpenMP <= 45 && CurrentRegion != OMPD_ordered) ||
+         (SemaRef.LangOpts.OpenMP >= 50 && CurrentRegion != OMPD_ordered &&
+          CurrentRegion != OMPD_simd && CurrentRegion != OMPD_atomic))) {
       // OpenMP [2.16, Nesting of Regions]
       // OpenMP constructs may not be nested inside a simd region.
       // OpenMP [2.8.1,simd Construct, Restrictions]
@@ -3808,9 +3811,14 @@ static bool checkNestingOfRegions(Sema &SemaRef, const DSAStackTy *Stack,
       // Allowing a SIMD construct nested in another SIMD construct is an
       // extension. The OpenMP 4.5 spec does not allow it. Issue a warning
       // message.
+      // OpenMP 5.0 [2.9.3.1, simd Construct, Restrictions]
+      // The only OpenMP constructs that can be encountered during execution of
+      // a simd region are the atomic construct, the loop construct, the simd
+      // construct and the ordered construct with the simd clause.
       SemaRef.Diag(StartLoc, (CurrentRegion != OMPD_simd)
                                  ? diag::err_omp_prohibited_region_simd
-                                 : diag::warn_omp_nesting_simd);
+                                 : diag::warn_omp_nesting_simd)
+          << (SemaRef.LangOpts.OpenMP >= 50 ? 1 : 0);
       return CurrentRegion != OMPD_simd;
     }
     if (ParentRegion == OMPD_atomic) {
@@ -8165,7 +8173,8 @@ StmtResult Sema::ActOnOpenMPOrderedDirective(ArrayRef<OMPClause *> Clauses,
     // OpenMP [2.8.1,simd Construct, Restrictions]
     // An ordered construct with the simd clause is the only OpenMP construct
     // that can appear in the simd region.
-    Diag(StartLoc, diag::err_omp_prohibited_region_simd);
+    Diag(StartLoc, diag::err_omp_prohibited_region_simd)
+        << (LangOpts.OpenMP >= 50 ? 1 : 0);
     ErrorFound = true;
   } else if (DependFound && (TC || SC)) {
     Diag(DependFound->getBeginLoc(), diag::err_omp_depend_clause_thread_simd)