[Orc] Add an emitAndFinalize method to the ObjectLinkingLayer, IRCompileLayer

and LazyEmittingLayer of Orc.

This method allows you to immediately emit and finalize a module. It is required
by an upcoming refactor of the indirection utils and the compile-on-demand
layer.

I've filed http://llvm.org/PR22608 to write unit tests for this and other Orc
APIs.

llvm-svn: 229451