[ThinLTO] Clang support for emitting minimized bitcode for thin link

Summary:
Clang companion patch to LLVM patch D31027, which adds support
for emitting minimized bitcode file for use in the thin link step.
Add a cc1 option -fthin-link-bitcode=<file> to trigger this behavior.

Depends on D31027.

Reviewers: mehdi_amini, pcc

Subscribers: cfe-commits, Prazek

Differential Revision: https://reviews.llvm.org/D31050

llvm-svn: 298639