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

[Eric Fiselier via cfe-commits]

Hi Adrian,

So the goal of this patch was to increase the coverage of the test
suite by default.
Previously we used C++11 as the default but this meant that new C++14
and C++1z tests
were never run.  The failures you are seeing are all C++14 tests.

We should work on actually fixing the failing tests. Reverting the
commit will only
hide these test failures again.

All of the errors seem to be caused by latent problems in how libc++ links to
libc++abi. on Apple platforms. libc++ always uses the system provided
libc++abi even
when it should use a newer version.

I *think* I know how to fix these issues but I would appreciate an Apple expert
to look at the fixes. Would you be able to help?

/Eric