[mlir][orc] unbreak MLIR ExecutionEngine after ORC changes

Changes to ORC in ce2207ab changed the
APIs in IRCompileLayer, now requiring the custom compiler to be wrapped
in IRCompileLayer::IRCompiler. Even though MLIR relies on Orc
CompileUtils, the type is still visible in several places in the code.
Adapt those to the new API.

(cherry picked from commit 7984b474)