C++ front-end: fix parentheses matching for alignas parsing

We had parentheses consumed by rCommaExpression while still expecting to
find one after this rule executed.

Author: tautschnig

regression/cpp/type_traits_essentials1/main.cpp

@@ -30,6 +30,11 @@ template<typename T> using x=V<T>;
 using type __attribute__((__nodebug__)) = int;
 #endif
 
+struct alignas(0x1) t
+{
+  char c;
+};
+
 template<typename T>
 class C
 {