glsl: Check that interpolation qualifiers do not precede 'varying'

... and 'centroid varying'. The check is performed only in GLSL
versions >= 1.30.

From page 29 (page 35 of the PDF) of the GLSL 1.30 spec:
   "interpolation qualifiers may only precede the qualifiers in, centroid
    in, out, or centroid out in a declaration. They do not apply to the
    deprecated storage qualifiers varying or centroid varying."

Fixes Piglit test
spec/glsl-1.30/compiler/interpolation-qualifiers/smooth-varying-01.frag.

src/glsl/ast.h

@@ -365,6 +365,11 @@ struct ast_type_qualifier {
     */
    unsigned location;
 
+   /**
+    * Return true if and only if an interpolation qualifier is present.
+    */
+   bool has_interpolation() const;
+
    /**
     * \brief Return string representation of interpolation qualifier.
     *