Make -std=c++17 an alias of -std=c++1z

As suggested on PR33912.

Trying to keep this small to make it easy to merge to the 5.0 branch. We
can do a follow-up with more thorough renaming (diagnostic text,
options, ids, etc.) later.

(For C++14 this was done in r215982, and I think a smaller patch for the
3.5 branch:
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20140818/113013.html)

Differential Revision: https://reviews.llvm.org/D36532

llvm-svn: 310516