[InlineCost] make InlineCost assignable

Summary:
Current InlineCost is not assignable because of const members Cost and Threshold.
I dont see practical benefits from having them const (access to these members is
private and internal interactions are rather simple). On other hand that makes
it hard to use as a member in some other data structure where assignability is necessary.

I'm going to use InlineCost in a downstream inliner that maintains a complex queue
of candidate call-sites and thus keeping and recalculating InlineCost is necessary.

This patch just removes 'const' from both members, making InlineCost assignable.

Reviewers: eraman, greened, chandlerc, yrouban, apilipenko
Reviewed By: apilipenko
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D63823

llvm-svn: 364612