[Inlining] Add PreInlineThreshold for the new pass manager

Summary:
This patch makes it easy to try out different preinlining thresholds
with a command-line switch just like -preinline-threshold for the
legacy pass manager.

Reviewers: davidxl

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72618