Fix compilation in C++17 mode.

C++17 removes `std::unexpected_handler`, but libc++abi needs it to define
`__cxa_exception`. When building against libc++, this is easily rectified by
telling libc++ we're building the library. We already do this in the other
places where we need these symbols.

Differential Revision: https://reviews.llvm.org/D42987

llvm-svn: 324542