[libcxx] Use newest supported language dialect when running the test suite.

Summary:
Currently the test suite defaults to C++11 mode if no standard version is supplied to LIT using `--param=std=c++XX`.  This patch changes that behavior so that the newest possible dialect is selected instead.

I have already patched the C++11 bot to explicitly specify `--param=std=c++11`. I'm just putting this up for review to see if anybody objects to this idea.

Reviewers: mclow.lists, jroelofs, danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D13331

llvm-svn: 249226