[mlir] [ExecutionEngine] add option to enable/disable GDB notification listener

Summary:
This way, clients can opt-out of the GDB notification listener. Also, this
changes the semantics of enabling the object cache, which seemed the wrong
way around.

Reviewers: rriddle, nicolasvasilache, ftynse, andydavis1

Reviewed By: nicolasvasilache

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75787