[python] Support PathLike filenames and directories

[python] Support PathLike filenames and directories

Python 3.6 introduced a file system path protocol (PEP 519[1]).
The standard library APIs accepting file system paths now accept path
objects too. It could be useful to add this here as well
for convenience.

[1] https://www.python.org/dev/peps/pep-0519

Authored by: jstasiak (Jakub Stasiak)

Differential Revision: https://reviews.llvm.org/D54120

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@346586 91177308-0d34-0410-b5e6-96231b3b80d8