[llvm][NFC] Add an explicit 'ComputeFullInlineCost' API

Summary:
Added getInliningCostEstimate, which is essentially what getInlineCost
computes if passed default inlining params, and  non-null ORE or
InlineParams::ComputeFullInlineCost.

Reviewers: davidxl, eraman, jdoerfert

Subscribers: hiraditya, haicheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78730