implement getfield overloading (#24960)
New functions `Base.getproperty` and `Base.setproperty!`
can be overloaded to change the behavior of `x.p` and `x.p = v`,
respectively.
This forces inference constant propagation through get/setproperty,
since it is very likely this method will yield better information after specializing on the field name
(even if `convert` is too big to make us want to inline the generic version and trigger the heuristic normally).
closes #16195 (and thus also closes #16226)
fix #1974
