[COFF] added test for thinlto

Summary: Creates bitcode files suitable for use with ThinLTO, then checks that the linker can build an executable from them.

Reviewers: ruiu, pcc

Reviewed By: pcc

Subscribers: mehdi_amini, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D30277

llvm-svn: 296042