glsl: fix 'shared' layout qualifier related regressions

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>