Linker utilities common to the GLSL IR and NIR linker (the latter to
be used for ARB_gl_spirv).
We need to move it to a new header as the NIR linker doesn't need to
know about ir_variable, and others, included at linker.h.
v2: move from src/compiler to src/compiler/glsl (Timothy Arceri)
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
