[ThinLTO] Fix ThinLTOCodegenerator to export llvm.used symbols

Summary:
ThinLTOCodeGenerator currently does not preserve llvm.used symbols and
it can internalize them. In order to pass the necessary information to the
legacy ThinLTOCodeGenerator, the input to the code generator is
rewritten to be based on lto::InputFile.

This fixes: PR41236
rdar://problem/49293439

Reviewers: tejohnson, pcc, dexonsmith

Reviewed By: tejohnson

Subscribers: mehdi_amini, inglorion, eraman, hiraditya, jkorous, dang, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60226

llvm-svn: 357931