[libcxx] Enable noexcept for GCC 4.6 and greater

Summary:
This patch allows GCC 4.6 and above to use `noexcept` as opposed to
`throw()`.

Is it an ABI safe change to suddenly switch on `noexcept`? I imagine
it must be because it's disabled in w/ clang in C++03 but not C++11.

Reviewers: danalbert, jroelofs, mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D15516