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
 {

src/cpp/parse.cpp

@@ -2115,9 +2115,8 @@ bool Parser::optAlignas(typet &cv)
 
   typet tname;
   cpp_tokent op, cp;
-
-  cpp_token_buffert::post pos=lex.Save();
   lex.get_token(op);
+  cpp_token_buffert::post pos = lex.Save();
 
   if(rTypeName(tname))
   {