[ThinLTO/gold] Add caching support to gold-plugin

Summary:
With support now in the new LTO API for caching (r279576), add
optional ThinLTO caching in the gold-plugin.

Reviewers: mehdi_amini

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D23836

llvm-svn: 279631