Add Hurd target to Clang driver (2/2)

This adds Hurd toolchain support to Clang's driver in addition
to handling translating the triple from Hurd-compatible form to
the actual triple registered in LLVM.

(Phabricator was stripping the empty files from the patch so I 
manually created them)

Patch by sthibaul (Samuel Thibault)

Differential Revision: https://reviews.llvm.org/D54379

llvm-svn: 347833