We are updating the Clang version on Ubuntu 16.04 bots to 4.0, because Clang < 4.0 is incompatible with libstdc++ 5.4 for at least LLVM’s usage of std::function. This will be required after we update the llvm-project branch. (Updating llvm-project swift/master branch)
How to install Clang-4.0 on Ubuntu 16.04:
$ apt install clang-4.0
$ update-alternatives --install /usr/bin/clang clang /usr/bin/clang-4.0 100
$ update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-4.0 100
Verify Clang version:
$ clang --version
$ clang++ --version