[llvm] Native size estimator for training -Oz inliner

Summary:
This is an experimental ML-based native size estimator, necessary for
computing partial rewards during -Oz inliner policy training. Data
extraction for model training will be provided in a separate patch.

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

Reviewers: davidxl, jdoerfert

Subscribers: mgorny, hiraditya, mgrang, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82817