[ThinLTO] Port InlinerFunctionImportStats handling to new PM

Summary:
The InlinerFunctionImportStats will collect and dump stats regarding how
many function inlined into the module were imported by ThinLTO.

Reviewers: wmi, dexonsmith

Subscribers: mehdi_amini, inglorion, llvm-commits, eraman

Differential Revision: https://reviews.llvm.org/D48729

llvm-svn: 335914