[llvm][NFC] Inliner: factor cost and reporting out of inlining process

Summary:
This factors cost and reporting out of the inlining workflow, thus
making it easier to reuse when driving inlining from the upcoming
InliningAdvisor.

Depends on: D79215

Reviewers: davidxl, echristo

Subscribers: eraman, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79275