glsl: fix 'shared' layout qualifier related regressions

Commit 8b28b35 added 'shared' as a keyword for compute shaders
but it broke the existing 'shared' layout qualifier support for
uniform and shader storage blocks.

This patch fixes 578 dEQP-GLES31.functional.ssbo.* tests.

v2:
- Move SHARED to interface_block_layout_qualifier (Timothy)
- Don't remove "shared" case insensitive check (Timothy)
- Remove the clearing of shared_storage flag (Timothy)

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>

src/glsl/glsl_parser.yy

@@ -1652,6 +1652,11 @@ interface_block_layout_qualifier:
       memset(& $$, 0, sizeof($$));
       $$.flags.q.packed = 1;
    }
+   | SHARED
+   {
+      memset(& $$, 0, sizeof($$));
+      $$.flags.q.shared = 1;
+   }
    ;
 
 subroutine_qualifier: