missing const checking for array of owned copy-init

[mppf]

This program should fail to compile

class C { var x: int; }

proc main() {
  var X: [1..2] owned C?;
  X[1] = new owned C(1);
  X[2] = new owned C(2);

  const Y = X;

  const Z = Y;

  writeln(X);
  writeln(Y);
  writeln(Z);
}

because the const Z = Y does an init-copy from Y which is const. When copying the elements, the RHS will be set to nil (because owned = does that).

The array init-copy should consider the case that the element type has an init= / = that mutates the RHS.

PR #14887 attempted to fix it but that caused problems with shadow variables with the test test test/parallel/forall/reduce-intents/ri-a2-AoA.chpl and so PR #14922 removed the change.

Associated Future Test

test/types/records/const-checking/array-of-owned-const.chpl (added in PR #14998).