[Orc] Add support for updating stub targets to CompileOnDemandLayer.

This makes it possible to implement re-optimization on top of the
CompileOnDemandLayer.

Test case to come in a future patch: This will need an execution test, and
execution tests require a full working stack. The best option is to plumb this
API up to the C Bindings stack and add a C bindings test for this.

Patch by Sean Ogden. Thanks Sean!

llvm-svn: 277257