This commit refactors validators-related logic that is common across most of the directives.
A couple notes on this refactoring:
* common logic was moved to the `AbstractControlDirective` class (including `validator` and
`asyncValidator` getters)
* sync/async validators are now composed in `AbstractControlDirective` class eagerly when validators
are set with `_setValidators` and `_setAsyncValidators` calls and the result is stored in directive
instance (thus getters return cached versions of validator fn). This is needed to make sure composed
validator function remains the same (retains its identity) for a given directive instance, so that
this function can be added and later removed from an instance of an AbstractControl-based class
(like `FormControl`). Preserving validator function is required to perform proper cleanup (in followup
PRs) of the AbstractControl-based classes when a directive is destroyed.
PR Close#38280
0723331b2a