Building Swift from source is a big pain in the *. I tried to build it on a shabby VPS running CentOS 7, which is not of my choice but because it was discounted. I had to setup 6GB of swap file to avoid OOM, and then found the intermediate files filled up the disk. Why does LLVM needs so much space to build?
So I switched to ninja (cmake -G Ninja). It can use all the cores and not run out of memory.