[lld-link] implement -thinlto-index-only

Summary:
This implements -thinlto-index-only, -thinlto-index-only:,
and -thinlto-emit-imports-files options in lld-link. They are
analogous to their counterparts in ld.lld: -thinlto-index-only
causes us to perform ThinLTO's thin link and write index files,
but not perform code generation. -thinlto-index-only: does the
same, but also writes a text file listing the native object
files expected to be generated. -thinlto-emit-imports-files
creates a text file next to each index file, listing the files
to import from.

Reviewers: ruiu, tejohnson, pcc, rnk

Subscribers: mehdi_amini, steven_wu, dexonsmith, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64461

llvm-svn: 365800