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
Sam McCall