[lli] Make -jit-kind=orc the default JIT engine

MCJIT served well as the default JIT engine in lli for a long time, but the code is getting old and maintenance efforts don't seem to be in sight. In the meantime Orc became mature enough to fill that gap. The newly added greddy mode is very similar to the execution model of MCJIT. It should work as a drop-in replacement for common JIT tasks.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D98931