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.