[lli] Don't create an ExecutorProcessControl in non-remote MCJIT config.

When lli is run with -jit-kind=mcjit and without remote JITing there is no need
to create an ExecutorProcessControl object.

This is nice cleanup, but as a bonus should silence the leak reports that we're
seeing in https://green.lab.llvm.org/green/job/clang-stage2-cmake-RgSan/9105/.
(Though as a side note I believe those reports are spurious. They are being
tracked by rdar://104825696).