Use a simpler set of mock headers for the vfs+modules crash recovery tests.

The System/ mock is large and too complex for this test. It can cause
the tests to fail in mysterious ways as it depends on the resource dir
being present, which is not really supported for driver tests (using
%clang instead of %clang_cc1). Copy the tree and trim out all the
%unnecessary fat.

llvm-svn: 263718