[llvm][NFC] Inliner: rename call site variables.

Summary:
Renamed 'CS' to 'CB', and, in one case, to a more specific name to avoid
naming collision with outer scope (a maintainability/readability reason,
not correctness)

Also updated comments.

Reviewers: davidxl, dblaikie, jdoerfert

Subscribers: eraman, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79101