[ThinLTO] Handle -fdebug-pass-manager for backend invocations via clang

Summary:
The LTO Config field wasn't being set when invoking a ThinLTO backend
via clang (i.e. for distributed builds).

Reviewers: danielcdh

Subscribers: mehdi_amini, inglorion, eraman, cfe-commits

Differential Revision: https://reviews.llvm.org/D39923

llvm-svn: 317951