[libcxx] Enable noexcept for GCC 4.6 and greater

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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@255683 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed