Introducing llvm-libtool-darwin

This diff starts the implementation of llvm-libtool-darwin
(an llvm based replacement of cctool's libtool).
Libtool is used for creating static and dynamic libraries
from a bunch of object files given as input.

Reviewed by alexshap, smeenai, jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D82923