Make filesystem tests generic between experimental and std versions.

As I move towards implementing std::filesystem, there is a need to
make the existing tests run against both the std and experimental versions.
Additionally, it's helpful to allow running the tests against other
implementations of filesystem.

This patch converts the test to easily target either. First, it
adds a filesystem_include.hpp header which is soley responsible
for selecting and including the correct implementation. Second,
it converts existing tests to use this header instead of including
filesystem directly.

llvm-svn: 328475