[llvm][Support] Take in CurrentDirectory as a parameter in ExpandResponseFiles

Summary:
This is a follow-up to D70769 and D70222, which allows propagation of
current directory down to ExpandResponseFiles for handling of relative paths.

Previously clients had to mutate FS to achieve that, which is not thread-safe
and can even be thread-hostile in the case of real file system.

Reviewers: sammccall

Subscribers: hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D70857