How to install libc++ 3.9 on Ubuntu Trusty without building from source

Question

My current attempt to install libc++ 3.9 on Ubuntu Trusty looks as follows:

wget -O - http://llvm.org/apt/llvm-snapshot.gpg.key|sudo apt-key add -
sudo add-apt-repository "deb http://llvm.org/apt/trusty/ llvm-toolchain.trusty-3.9 main"
sudo apt-get update
sudo apt-get install clang-3.9 lldb-3.9

My main.cpp is this:

// main.cpp
#include <iostream>
int main()
{
    std::cout << _LIBCPP_VERSION << std::endl;
}

But compiling and running it

clang++-3.9 -stdlib=libc++ main.cpp 
./a.out

prints

1101

which is the version number for libc++ svn199600, the default one provided on Trusty.

How can I update to version 3.9 without building from source?