Add comment to clarify why we don't use copySymbol

src/compiler/checker.ts

@@ -12680,6 +12680,10 @@ namespace ts {
                             break;
                         case SyntaxKind.FunctionExpression:
                         case SyntaxKind.ClassExpression:
+                            // The reason we are not using copySymbol for function expression and class expression
+                            // is that copySymbol will not copy a symbol into SymbolTable if the symbol has name prefix
+                            // with "__". Therefore, if class expression or function expression have declared name,
+                            // we will add the symbol into the table using its declared name
                             let name = (<FunctionExpression|ClassExpression>location).name;
                             if (name) {
                                 let symbol = location.symbol;