lalr1.cc: modern C++ no longer needs an assignment for symbols

Reported by Frank Heckenbach.
http://lists.gnu.org/archive/html/bug-bison/2018-03/msg00002.html

Actually the assignment operator should never be needed: the C++98
requirements for vector::push_back is CopyInsertable, which does not require
an assignment operator.  However, libstdc++ shipped with GCC up to (and
including) 6 uses the assignment operator (which affects Clang on top of
libstdc++, but also ICC).  So let's keep it for legacy C++.

See https://gcc.godbolt.org/z/q0XXmC.

* data/lalr1.cc (stack_symbol_type::operator=): Remove.
* data/c++.m4 (basic_symbol::operator=): Ditto.
* tests/c++.at (C++ Variant-based Symbols Unit Tests): Adjust.

tests/c++.at

@@ -166,12 +166,12 @@ int main()
     for (int i = 0; i < 100; ++i)
       {
 #if defined __cplusplus && 201103L <= __cplusplus
-        auto ss = parser::stack_symbol_type(1, parser::make_INT(123));
+        st.push(parser::stack_symbol_type{1, parser::make_INT(123)});
 #else
         parser::symbol_type s = parser::make_INT(123);
         parser::stack_symbol_type ss(1, s);
-#endif
         st.push(ss);
+#endif
       }
   }
 }