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