[libcxx] [test] Ifdef out uses of create_fifo on windows

Restructure code in directory_entry.obs/file_type_obs.pass.cpp
and directory_entry.obs/hard_link_count.pass.cpp to reduce the
amount of ifdeffery needed.

In file_type_obs.pass.cpp, we can't inline the calls to
env.create_* into the lambda calls (e.g. "test_path(env.create_*())"),
because the lambda removes the referenced file, and the hardlink
must be created while the earlier test file exists.

In hard_link_count.pass.cpp, move restoration of the original
directory permissions to the end of the lambda, so that new
directory entries can be created after the lambda has run once.

Differential Revision: https://reviews.llvm.org/D89948