[ORC] Add definition for IRLayer::setCloneToNewContextOnEmit, use it to set the

flag to true in LLJIT when running in multithreaded mode.

The IRLayer::setCloneToNewContextOnEmit method sets a flag within the IRLayer
that causes modules added to that layer to be moved to a new context (by
serializing to/from a memory buffer) when they are emitted. This allows modules
that were all loaded on the same context to be compiled in parallel.

llvm-svn: 343266