Implement national body comment GB9: remove std::gets

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@192538 91177308-0d34-0410-b5e6-96231b3b80d8
LWG issue 2143: ios_base::xalloc should be thread-safe

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@192539 91177308-0d34-0410-b5e6-96231b3b80d8