[clang][ThinLTO] Promote cc1 -fthin_link_bitcode to driver -fthinlto_link_bitcode

Summary:
A necessary step to let build system caching work for its output.

Reviewers: tejohnson, steven_wu

Reviewed by: tejohnson

Subscribers: mehdi_amini, inglorion, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69406