[Orc][lli] Add a very simple Orc-based lazy JIT to lli.

This ensures that we're building and testing the CompileOnDemand layer, at least
in a basic way.

Currently x86-64 only, and with limited to no library calls enabled (depending
on host platform). Patches welcome. ;)

To enable access to the lazy JIT, this patch replaces the '-use-orcmcjit' lli
option with a new option:
'-jit-kind={ mcjit | orc-mcjit | orc-lazy }'.

All regression tests are updated to use the new option, and one trivial test of
the new lazy JIT is added.

llvm-svn: 233182