Add an option to support debug fission on implicit ThinLTO.

Summary:
This adds an option -gsplit-dwarf=<arg>. LLVM can create .dwo files in the given directory
during the implicit ThinLTO link stage.

Reviewers: tejohnson, dblaikie, pcc

Reviewed By: pcc

Subscribers: steven_wu, aprantl, JDevlieghere, yunlian, probinson, mehdi_amini, inglorion, cfe-commits

Differential Revision: https://reviews.llvm.org/D44788