Bleeding-edge LLVM has stopped supplying replacements for various
C++14 library features, for people on older C++ versions.  Since we're
not ready to require C++14 yet, just use plain old new instead of
make_unique.  As revealed by buildfarm animal seawasp.

Back-patch to 11.

Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/CA%2BhUKGJWG7unNqmkxg7nC5o3o-0p2XP6co4r%3D9epqYMm8UY4Mw%40mail.gmail.com