[NewPM][Pipeline] Add PipelineTuningOption to set inliner threshold

The legacy PM allowed you to set a custom inliner threshold via
  builder.Inliner = llvm::createFunctionInliningPass(inline_threshold);

This allows the same thing to be done with the new PM optimization pipelines.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D137038