[lldb] Use OrcMCJITReplacement rather than MCJIT as the underlying JIT for LLDB

expression evaluation.

OrcMCJITReplacement is a reimplementation of MCJIT using ORC components, and
provides an easy upgrade path to ORC for existing MCJIT clients. There should be
no functional changes resulting from this switch.

llvm-svn: 279327