compiler/glsl: refactor empty_uniform_block utilities to linker_util

compiler/glsl: refactor empty_uniform_block utilities to linker_util

This includes:
  * Move the defition of empty_uniform_block to linker_util.h
  * Move find_empty_block (with a rename) to linker_util.h
  * Refactor some code at linker.cpp to a new method at linker_util.h
    (link_util_update_empty_uniform_locations)

So all that code could be used by the GLSL linker and the NIR linker
used for ARB_gl_spirv.

v2: include just "ir_uniform.h" (Timothy Arceri)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>