Revert my previous change, got stupidly confused with my local changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57514 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Sema/SemaDeclCXX.cpp

@@ -510,7 +510,7 @@ Sema::DeclTy *Sema::ActOnStartNamespaceDef(Scope *NamespcScope,
     // in that declarative region, it is treated as an original-namespace-name.
 
     Decl *PrevDecl =
-        LookupDecl(II, Decl::IDNS_Tag | Decl::IDNS_Ordinary, DeclRegionScope, 0,
+        LookupDecl(II, Decl::IDNS_Tag | Decl::IDNS_Ordinary, DeclRegionScope,
                    /*enableLazyBuiltinCreation=*/false);
 
     if (PrevDecl && isDeclInScope(PrevDecl, CurContext, DeclRegionScope)) {