As is suggested in issue #15257, the function preimage is a general
solution to the optimization problem with predicates containing the
date and datetime converters. This commit implements the idea by
adding the new methods, hasInformationAboutPreimage and getPreimage,
to IFunction/IFunctionBase, and having the specific convert functions
define their own preimage. Moreover, we added a new pass in the
TreeOptimizer and a new AST visitor for in-place rewriting the AST
with the converters' preimage.
Specifically, the optimization is applied to toYear and toYYYYMM.
