[InlineCost] Add flag to allow changing the default inline cost

Summary:
It can be useful to tune the default inline threshold without overriding other inlining thresholds (e.g. in code compiled for size).

The existing `-inline-threshold` flag overrides other thresholds, so it is insufficient in codebases where there is a mix of code compiled for size and speed.

Patch by Michael Holman <michael.holman@microsoft.com>

Reviewers: eraman, tejohnson

Reviewed By: tejohnson

Subscribers: tejohnson, mtrofin, davidxl, hiraditya, haicheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73217