Merge pull request #32747 from chsienki/box_ref_check

Assert when emitting a box IL opcode for ref like type

src/Compilers/CSharp/Portable/CodeGen/EmitExpression.cs

@@ -3359,6 +3359,8 @@ private void EmitStaticCast(TypeSymbol to, SyntaxNode syntax)
 
         private void EmitBox(TypeSymbol type, SyntaxNode syntaxNode)
         {
+            Debug.Assert(!type.IsRefLikeType);
+
             _builder.EmitOpCode(ILOpCode.Box);
             EmitSymbolToken(type, syntaxNode);
         }