Change the default value for `preloadDialectsInContext` for MlirOptMain

Authored by mehdi_amini on Wed, Apr 7, 8:46 PM.


Change the default value for preloadDialectsInContext for MlirOptMain

This option has been deprecated for 6 months, change the default setting for now before
future removal.

While clients can set the option to true for now, they should start
updating their passes to define the right dependentDialects in
preparation of the removal of this option. See the FAQ for more info:

Reviewed By: rriddle, jpienaar

Differential Revision: https://reviews.llvm.org/D99025