CodeGen: Replace ThinLTO backend implementation with a client of LTO/Resolution.

Summary:
This changes clang to use the llvm::lto::thinBackend function instead of
its own less comprehensive ThinLTO backend implementation.

Patch by Peter Collingbourne

Reviewers: tejohnson, mehdi_amini

Subscribers: cfe-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D21545

llvm-svn: 278541