implement more of N4258 - Cleaning up noexcept in the standard library. Specifically add new noexcept stuff to vector and string's move-assignment operations

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@245330 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed