glsl: Properly handle .length() of an unsized array

glsl: Properly handle .length() of an unsized array

There are two distinct cases:
- The last member of a shader storage block (length determined at run-time)
- Implicitly-sized array (length determined at link-time)

Fixes: 273f61a0051a ("glsl: Add parser/compiler support for unsized array's length()")
Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11952>