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.

data/c++.m4

@@ -272,8 +272,10 @@ m4_define([b4_symbol_type_declare],
       location_type location;])[
 
     private:
+#if defined __cplusplus && __cplusplus < 201103L
       /// Assignment operator.
       basic_symbol& operator= (const basic_symbol& other);
+#endif
     };
 
     /// Type access provider for token (enum) based symbols.

data/lalr1.cc

@@ -326,8 +326,10 @@ b4_location_define])])[
       stack_symbol_type (YY_RVREF (stack_symbol_type) that);
       /// Steal the contents from \a sym to build this.
       stack_symbol_type (state_type s, YY_MOVE_REF (symbol_type) sym);
-      /// Assignment, needed by push_back.
+#if defined __cplusplus && __cplusplus < 201103L
+      /// Assignment, needed by push_back by some old implementations.
       stack_symbol_type& operator= (YY_MOVE_REF (stack_symbol_type) that);
+#endif
     };
 
     /// Stack type.
@@ -608,6 +610,7 @@ m4_if(b4_prefix, [yy], [],
     that.type = empty_symbol;
   }
 
+#if defined __cplusplus && __cplusplus < 201103L
   ]b4_parser_class_name[::stack_symbol_type&
   ]b4_parser_class_name[::stack_symbol_type::operator= (YY_MOVE_REF (stack_symbol_type) that)
   {
@@ -618,6 +621,7 @@ m4_if(b4_prefix, [yy], [],
     location = YY_MOVE (that.location);])[
     return *this;
   }
+#endif
 
   template <typename Base>
   void

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
       }
   }
 }