[mlir] ExecutionEngine: default enableObjectCache to false

The enableObjectCache option was added in
https://reviews.llvm.org/rG06e8101034e, defaulting to false. However,
the init code added there got its logic reversed
(cache(enableObjectCache ? nullptr : new SimpleObjectCache()), which was
fixed in https://reviews.llvm.org/rGd1186fcb04 by setting the default to
true, thereby preserving the existing behavior even if it was
unintentional.

Default now the object cache to false as it was originally intended.
While at it, mention in enableObjectCache's documentation how the
cache can be dumped.

Reviewed-by: mehdi_amini
Differential Revision: https://reviews.llvm.org/D121291