[Reproducers] Make clang use lldb's VFS.

Authored by JDevlieghere on Feb 18 2019, 12:31 PM.


[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

llvm-svn: 354283