glsl: Add an is_declaration field to ast_struct_specifier.

Will be used in a later commit to differentiate between a structure type
declaration and a variable declaration of a struct type. I.e., the
difference between

   struct S { float x; }; (is_declaration = true)

and

   S s;                   (is_declaration = false)

Also note that is_declaration = true for

   struct S { float x; } s;

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ast.h

@@ -460,7 +460,8 @@ public:
     * be modified. Zeros the inherited ast_node's fields.
     */
    ast_struct_specifier(const ast_struct_specifier& that):
-      ast_node(), name(that.name), declarations(that.declarations)
+      ast_node(), name(that.name), declarations(that.declarations),
+      is_declaration(that.is_declaration)
    {
       /* empty */
    }
@@ -475,6 +476,7 @@ public:
    const char *name;
    /* List of ast_declarator_list * */
    exec_list declarations;
+   bool is_declaration;
 };