glsl: Add assert for unhandled ir_shader case.

Silences this GCC warning.
ast_to_hir.cpp: In function 'void apply_type_qualifier_to_variable(const
ast_type_qualifier*, ir_variable*, _mesa_glsl_parse_state*, YYLTYPE*)'
ast_to_hir.cpp:1768: warning: enumeration value 'ir_shader' not handled
in switch

src/glsl/ast_to_hir.cpp

@@ -1785,6 +1785,10 @@ apply_type_qualifier_to_variable(const struct ast_type_qualifier *qual,
 	    string = "output";
 	 }
 	 break;
+
+      case ir_shader:
+	 assert(!"Should not get here.");
+	 break;
       }
 
       if (fail) {