postinst is in llvm-3.9-runtime 1:3.9.1-19ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9  | #!/bin/sh
set -e
if test "$1" = "configure"; then
    if test -x /usr/sbin/update-binfmts; then
        update-binfmts --import llvm-3.9-runtime.binfmt || true
    fi
fi
 |