compiler/link: add linker_util.h, move linker_error/warning to it

compiler/link: add linker_util.h, move linker_error/warning to it

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>