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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229451 91177308-0d34-0410-b5e6-96231b3b80d8