Add support for GNU Hurd in Path.inc and other places

Summary: Patch by Svante Signell & myself

Reviewers: rnk, JDevlieghere, efriedma

Reviewed By: efriedma

Subscribers: efriedma, JDevlieghere, krytarowski, llvm-commits, kristina

Differential Revision: https://reviews.llvm.org/D53409

llvm-svn: 345007