Add a GDB/LLDB interface for interactive debugging of MLIR Actions

This includes a small runtime acting as callback for the ExecutionEngine
and a C API that makes it possible to control from the debugger.

A python script for LLDB is included that hook a new `mlir` subcommand
and allows to set breakpoints and inspect the current action, the context
and the stack.

Differential Revision: https://reviews.llvm.org/D144817