Enable support for the [[nodiscard]] attribute from WG14 N2050 when enabling double square bracket attributes in C code.

llvm-svn: 316026
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 9b0d976..f27df1d 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -3056,7 +3056,8 @@
 const Attr *FunctionDecl::getUnusedResultAttr() const {
   QualType RetType = getReturnType();
   if (RetType->isRecordType()) {
-    if (const CXXRecordDecl *Ret = RetType->getAsCXXRecordDecl()) {
+    if (const auto *Ret =
+            dyn_cast_or_null<RecordDecl>(RetType->getAsTagDecl())) {
       if (const auto *R = Ret->getAttr<WarnUnusedResultAttr>())
         return R;
     }