[llvm-lto] Remove support for legacy pass manager

This removes support for the legacy pass manager in llvm-lto and
llvm-lto2. In this case I've dropped the use-new-pm option entirely,
as I don't think this is considered part of the public interface.

This also makes -debug-pass-manager work with llvm-lto, because
that was needed to migrate some tests to NewPM.

Differential Revision: https://reviews.llvm.org/D123376