glsl: find the base offset for block members from unnamed blocks

glsl: find the base offset for block members from unnamed blocks

These block member have been split into individual vars so we need
to set the correct offsets for each member in the new glsl nir
linker. We also take this opportunity to set the correct location
for the variable.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4050>