[OpenMP] Fix data sharing analysis in nested clause

[jdenny]

Summary

Without this patch, clang doesn't complain that X needs explicit data
sharing attributes in the following:

#pragma omp target teams default(none)
{
  #pragma omp parallel num_threads(X)
    ;
}

However, clang does produce that complaint after the braces are
removed. With this patch, clang complains in both cases.

[ABataev]

LG

lib/Sema/SemaOpenMP.cpp

for (OMPClause *C : S->clauses()) {
       }
     }
     // Check implicitly captured variables.
     VisitSubCaptures(S);
   }
   void VisitStmt(Stmt *S) {
     for (Stmt *C : S->children()) {
       if (C) {
-        if (auto *OED = dyn_cast<OMPExecutableDirective>(C)) {
-          // Check implicitly captured variables in the task-based directives to
-          // check if they must be firstprivatized.
-          VisitSubCaptures(OED);
-        } else {
-          Visit(C);
-        }
-      }
-    }
-  }
 
   bool isErrorFound() const { return ErrorFound; }
   ArrayRef<Expr *> getImplicitFirstprivate() const {
     return ImplicitFirstprivate;
   }
   ArrayRef<Expr *> getImplicitMap() const { return ImplicitMap; }
   const Sema::VarsWithInheritedDSAType &getVarsWithInheritedDSA() const {
     return VarsWithInheritedDSA;

test/OpenMP/target_teams_messages.cpp

case (0):
       }
       default:
        break;
     }
   }
 #pragma omp target teams default(none)
   ++argc; // expected-error {{variable 'argc' must have explicitly specified data sharing attributes}}
 
+#pragma omp target teams default(none)
+#pragma omp parallel num_threads(argc) // expected-error {{variable 'argc' must have explicitly specified data sharing attributes}}
+  ;
+
+#pragma omp target teams default(none)
+  {
+#pragma omp parallel num_threads(argc) // expected-error {{variable 'argc' must have explicitly specified data sharing attributes}}
+    ;
+  }
+
   goto L2; // expected-error {{use of undeclared label 'L2'}}
 #pragma omp target teams
   L2:
   foo();
 #pragma omp target teams
   {
     return 1; // expected-error {{cannot return from OpenMP region}}
   }
 
   [[]] // expected-error {{an attribute list cannot appear here}}
 #pragma omp target teams
   for (int n = 0; n < 100; ++n) {}
 
   return 0;
 }