glsl: rename _restrict to restrict_flag

To fix MSVC compile breakage.  Evidently, _restrict is an MSVC keyword,
though the docs only mention __restrict (with two underscores).

Note: we may want to also rename _volatile to volatile_flag to be
consistent.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74900
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ast_function.cpp

@@ -120,7 +120,7 @@ verify_image_parameter(YYLTYPE *loc, _mesa_glsl_parse_state *state,
       return false;
    }
 
-   if (actual->data.image._restrict && !formal->data.image._restrict) {
+   if (actual->data.image.restrict_flag && !formal->data.image.restrict_flag) {
       _mesa_glsl_error(loc, state,
                        "function call parameter `%s' drops "
                        "`restrict' qualifier", formal->name);