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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229461 91177308-0d34-0410-b5e6-96231b3b80d8