Disallow null as a named metadata operand.

Make MDNode::destroy private.
Fix the one thing that used MDNode::destroy, outside of MDNode itself.

One should never delete or destroy an MDNode explicitly. MDNodes
implicitly go away when there are no references to them (implementation
details aside).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109028 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AsmParser/LLParser.cpp

@@ -546,12 +546,6 @@ bool LLParser::ParseNamedMetadata() {
   SmallVector<MDNode *, 8> Elts;
   if (Lex.getKind() != lltok::rbrace)
     do {
-      // Null is a special case since it is typeless.
-      if (EatIfPresent(lltok::kw_null)) {
-        Elts.push_back(0);
-        continue;
-      }
-
       if (ParseToken(lltok::exclaim, "Expected '!' here"))
         return true;