[libc++] Split sources for <filesystem>

The operations.cpp file contained the implementation of a ton of
functionality unrelated to just the filesystem operations, and
filesystem_common.h contained a lot of unrelated functionality as well.

Splitting this up into more files will make it possible in the future
to support parts of <filesystem> (e.g. path) on systems where there is
no notion of a filesystem.

Differential Revision: https://reviews.llvm.org/D152377