[Reproducers] Make clang use lldb's VFS.

In r353906 we hooked up clang and lldb's reproducer infrastructure to
capture files used by clang. This patch adds the necessary logic to have
clang reuse the files from lldb's reproducer during replay.

Differential revision: https://reviews.llvm.org/D58309

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@354283 91177308-0d34-0410-b5e6-96231b3b80d8
15 files changed