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

src/cluster.jl

@@ -474,7 +474,7 @@ function launch_n_additional_processes(manager, frompid, fromconfig, cnt, launch
 
             wconfig = WorkerConfig()
             for x in [:host, :tunnel, :sshflags, :exeflags, :exename, :enable_threaded_blas]
-                setfield!(wconfig, x, getfield(fromconfig, x))
+                Base.setproperty!(wconfig, x, Base.getproperty(fromconfig, x))
             end
             wconfig.bind_addr = bind_addr
             wconfig.port = port