Avoid Clang error about throwing _LIBCPP_ASSERT in noexcept function.

This fixes a couple of tests which produced a warning that a 'throw'
occurred in a noexcept function (by way of _LIBCPP_ASSERT). It does
so by hiding the 'throw' across an opaque function boundary.

This fix isn't ideal, since we still have _LIBCPP_ASSERT's in functions
marked noexcept -- and this problem should be addressed in the future.
However, throwing _LIBCPP_ASSERT is really only meant to allow testing
of the assertions, and is not yet ready for general use.

llvm-svn: 328265