Remove `using namespace std;` in __gnu_cxx namespace.

The `using namespace std;` opens us up to ambiguity
when any of the std:: names are also present in the global namespace.
Instead we should properly qualify names we use from std::.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@361074 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed