[Driver] Use VFS to check if sanitizer blacklists exist

Summary:
This is a follow-up to 590f279c, which
moved some of the callers to use VFS.

It turned out more code in Driver calls into real filesystem APIs and
also needs an update.

Reviewers: gribozavr2, kadircet

Reviewed By: kadircet

Subscribers: ormris, mgorny, hiraditya, llvm-commits, jkorous, cfe-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D70440