[OPENMP]Fix PR49468: Declare target should allow empty sequences and namespaces.

[ABataev]

Summary

The emty declare target/end declare target region should not cause an
error emission.

[jdoerfert]

LGTM, thanks

clang/lib/Parse/ParseOpenMP.cpp

case OMPD_declare_target: {
     // Skip the last annot_pragma_openmp_end.
     ConsumeAnyToken();
 
     if (!Actions.ActOnStartOpenMPDeclareTargetDirective(DTLoc))
       return DeclGroupPtrTy();
 
     ParsingOpenMPDirectiveRAII NormalScope(*this, /*Value=*/false);
     llvm::SmallVector<Decl *, 4> Decls;
+    while (Tok.isNot(tok::eof) && Tok.isNot(tok::r_brace)) {
+      if (Tok.isAnnotation() && Tok.is(tok::annot_pragma_openmp)) {
+        TentativeParsingAction TPA(*this);
+        ConsumeAnnotationToken();
         DKind = parseOpenMPDirectiveKind(*this);
-    while (DKind != OMPD_end_declare_target && Tok.isNot(tok::eof) &&
-           Tok.isNot(tok::r_brace)) {
+        if (DKind != OMPD_end_declare_target)
+          TPA.Revert();
+        else
+          TPA.Commit();
+      }
+      if (DKind == OMPD_end_declare_target)
+        break;
       DeclGroupPtrTy Ptr;
       // Here we expect to see some function declaration.
       if (AS == AS_none) {
         assert(TagType == DeclSpec::TST_unspecified);
         MaybeParseCXX11Attributes(Attrs);
         ParsingDeclSpec PDS(*this);
         Ptr = ParseExternalDeclaration(Attrs, &PDS);
       } else {
         Ptr =
             ParseCXXClassMemberDeclarationWithPragmas(AS, Attrs, TagType, Tag);
       }
       if (Ptr) {
         DeclGroupRef Ref = Ptr.get();
         Decls.append(Ref.begin(), Ref.end());
       }
-      if (Tok.isAnnotation() && Tok.is(tok::annot_pragma_openmp)) {
-        TentativeParsingAction TPA(*this);
-        ConsumeAnnotationToken();
-        DKind = parseOpenMPDirectiveKind(*this);
-        if (DKind != OMPD_end_declare_target)
-          TPA.Revert();
-        else
-          TPA.Commit();
-      }
     }
 
     ParseOMPEndDeclareTargetDirective(DKind, DTLoc);
     Actions.ActOnFinishOpenMPDeclareTargetDirective();
     return Actions.BuildDeclaratorGroup(Decls);
   }
   case OMPD_unknown:
     Diag(Tok, diag::err_omp_unknown_directive);

clang/test/OpenMP/declare_target_ast_print.cpp

int main (int argc, char **argv) {
   baz<int>();
   return (0);
 }
 
 // CHECK: #pragma omp declare target
 // CHECK-NEXT: int ts = 1;
 // CHECK-NEXT: #pragma omp end declare target
 
+// Do not expect anything here since the region is empty.
+#pragma omp declare target
+#pragma omp end declare target
+
 #endif