glsl: Use 'using' to be explicit about visitor overloads

Clang has a warning about overloading virtuals that triggers when a
derived class defines a virtual function that's an overload of
function in the base class.  This kind of thing:

  struct chart; // let's pretend this exists
  struct Base
  {
      virtual void* get(char* e);
  };

  struct Derived: public Base {
      virtual void* get(chart* e); // typo, we wanted to override the same function
  };

The solution is to use

  using Base::get;

to be explicit about the intention to reuse the base class virtual.
We hit this a lot with out glsl ir_hierarchical_visitor visitor
pattern, so let's adds some 'using' to calm down the compiler.

See-also: https://stackoverflow.com/questions/18515183/c-overloaded-virtual-function-warning-by-clang)
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3686>

src/compiler/glsl/opt_constant_variable.cpp

@@ -49,6 +49,9 @@ struct assignment_entry {
 
 class ir_constant_variable_visitor : public ir_hierarchical_visitor {
 public:
+   using ir_hierarchical_visitor::visit;
+   using ir_hierarchical_visitor::visit_enter;
+
    virtual ir_visitor_status visit_enter(ir_dereference_variable *);
    virtual ir_visitor_status visit(ir_variable *);
    virtual ir_visitor_status visit_enter(ir_assignment *);