[llvm] Add interface to drive inlining decision using ML model

Summary:

This change introduces InliningAdvisor (and related APIs), the interface
that abstracts decision making away from the inlining pass. We will use
this interface to delegate decision making to a trained ML model,
subsequently (see referenced RFC).

RFC: http://lists.llvm.org/pipermail/llvm-dev/2020-April/140763.html

Reviewers: davidxl, eraman, dblaikie

Subscribers: mgorny, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79042