glsl: Move link_get_main_function_signature to a common location

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>