Libc++ tests failing with bad_cast


I'm building Clang with libc++ again on ARMv7 to see if we can
progress on moving them to the bots, and I'm getting failures on all
of libc++ tests of the same error:

    terminating with uncaught exception of type std::bad_cast: std::bad_cast

I'm compiling with Clang 3.8, bfd linker 2.26, glibc 2.23.

I have libstdc++6 (from GCC 5.4) installed and I'm doing a single build+test.

Should I build libc++ on the first stage and build stage 2 with libc++
in order for the tests to pass on stage 2?

We have the stand alone buildbot libc++ tests for both ARM and
AArch64, but it would be good to have them building directly in-tree,
so we can do a proper release and not worry about the failures.