[ThinLTO] Handle -emit-llvm* in ThinLTO backends

Summary:
Use PreCodeGenModuleHook to invoke the correct writer when emitting LLVM
IR, returning false to skip codegen from within thinBackend.

Reviewers: pcc, mehdi_amini

Subscribers: Prazek, cfe-commits

Differential Revision: https://reviews.llvm.org/D31534

llvm-svn: 299274