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/alignas1/main.cpp

@@ -0,0 +1,9 @@
+struct alignas(0x1) t
+{
+  char c;
+};
+
+int main(int argc, char* argv[])
+{
+  struct t t;
+}

regression/cpp/alignas1/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.cpp
+-std=c++11
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/cpp/parse.cpp

@@ -2151,9 +2151,8 @@ 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))
   {