glsl/linker: produce error when invalid explicit locations are used

glsl/linker: produce error when invalid explicit locations are used

We only need to add a check to validate output locations here. For
inputs with invalid locations we will fail to link when we can't
find a matching output in the same (invalid) location.

v2: compute location slots properly depending on shader stage and
    variable type / direction

Fixes:
KHR-GL45.enhanced_layouts.varying_location_limit

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>