Support option -plugin-opt=dwo_dir=

Summary:
This adds support to option -plugin-opt=dwo_dir=${DIR}. This option is used to specify the directory to store the .dwo files when LTO and debug fission is used
at the same time.

Reviewers: ruiu, espindola, pcc

Reviewed By: pcc

Subscribers: eraman, dexonsmith, mehdi_amini, emaste, arichardson, steven_wu, llvm-commits

Differential Revision: https://reviews.llvm.org/D47904