[llvm] Add function feature extraction analysis

Summary:
This patch introduces an analysis pass to extract function features,
which will be needed by the ML InlineAdvisor.

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

Reviewers: davidxl, dblaikie, jdoerfert

Subscribers: mgorny, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80579