[Orc] Update the Orc indirection utils and refactor the CompileOnDemand layer.

This patch replaces most of the Orc indirection utils API with a new class:
JITCompileCallbackManager, which creates and manages JIT callbacks.
Exposing this functionality directly allows the user to create callbacks that
are associated with user supplied compilation actions. For example, you can
create a callback to lazyily IR-gen something from an AST. (A kaleidoscope
example demonstrating this will be committed shortly).

This patch also refactors the CompileOnDemand layer to use the
JITCompileCallbackManager API.

llvm-svn: 229461