glsl: Move link_get_main_function_signature to a common location

Also rename to _mesa_get_main_function_signature.

We will call it near the end of compilation to insert some code into
main for initializing some compute shader global variables.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>

src/glsl/ir.h

@@ -2529,6 +2529,9 @@ _mesa_glsl_find_builtin_function_by_name(const char *name);
 extern gl_shader *
 _mesa_glsl_get_builtin_function_shader(void);
 
+extern ir_function_signature *
+_mesa_get_main_function_signature(gl_shader *sh);
+
 extern void
 _mesa_glsl_release_functions(void);