libc++ and const expr.

I had this problem (static_assert in include/ratio) with Clang HEAD a few days ago, but it has since been resolved (r150103). libc++ HEAD builds cleanly for me on Linux.

For the record, the ultimate fix for this issue was Richard Smith's r150059.

- David

Yes, the C++ core working group has (provisionally) agreed to make certain overflowing signed left shifts implementation-defined as the resolution of DR1457. Clang implemented support for this in r150059.