[libcxx] r249226 - [libcxx] Use newest supported language dialect when running the test suite.

[Eric Fiselier via cfe-commits]

Hi Adrian,

The point of this commit was to flush out hidden failures and it's clearly
working as intended. I don't want to revert this because it did it's
job. There are better
ways to handle expected test failures. If I can't put a fix together
tonight I will mark the tests
as XFAIL.

/Eric