glsl: Rename ubo_qualifiers_valid to ubo_qualifiers_allowed.

The variable means that UBO qualifiers are allowed in a particular
context (e.g., not allowed in a struct field declaration), rather than a
particular set of UBO qualifiers are valid.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/glsl/ast.h

@@ -644,7 +644,7 @@ public:
     * Flag indicating that these declarators are in a uniform block,
     * allowing UBO type qualifiers.
     */
-   bool ubo_qualifiers_valid;
+   bool ubo_qualifiers_allowed;
 };