glsl: fix subroutine mismatch between declarations/definitions

Previously, when q.subroutine was set to 1, a new subroutine
declaration was added to the AST, while 0 meant a subroutine
definition has been detected by the parser.

Thus, setting the q.subroutine flag in both situations is
obviously wrong because a new type identifier is added instead
of trying to match the declaration. To fix it up, introduce
ast_type_qualifier::is_subroutine_decl() to differentiate
declarations and definitions easily.

This fixes a regression with:
arb_shader_subroutine/compiler/direct-call.vert

Cc: Mark Janes <mark.a.janes@intel.com>
Fixes: be8aa76afd ("glsl: remove unecessary flags.q.subroutine_def")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100026
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/ast.h

@@ -770,6 +770,11 @@ struct ast_type_qualifier {
     */
    bool has_memory() const;
 
+   /**
+    * Return true if the qualifier is a subroutine declaration.
+    */
+   bool is_subroutine_decl() const;
+
    bool merge_qualifier(YYLTYPE *loc,
 			_mesa_glsl_parse_state *state,
                         const ast_type_qualifier &q,