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>

src/compiler/glsl/program.h

@@ -48,14 +48,6 @@ extern void
 build_program_resource_list(struct gl_context *ctx,
                             struct gl_shader_program *shProg);
 
-extern void
-linker_error(struct gl_shader_program *prog, const char *fmt, ...)
-   PRINTFLIKE(2, 3);
-
-extern void
-linker_warning(struct gl_shader_program *prog, const char *fmt, ...)
-   PRINTFLIKE(2, 3);
-
 extern long
 parse_program_resource_name(const GLchar *name,
                             const GLchar **out_base_name_end);