add -fthinlto-index= option to clang-cl

Summary:
This adds a -fthinlto-index= option to clang-cl, which allows it to
be used to drive ThinLTO backend passes. This allows clang-cl to be
used for distributed ThinLTO.

Reviewers: tejohnson, pcc, rnk

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64458

llvm-svn: 366146