fixes #24806; generates =wasMoved for seqs self-assignment to reset the buffer

[ringabout]

fixes #24806

In the case of self assignments, x.p is the same as y.p. We cannot modify x.p without changing y.p as well. We need to reset x.p to nil so that setLen will initialize x with uninit values. Now it's okay to copy values of y.p

proc `=copy`(x, y: seq) =
  if x.p == y.p:
    wasMoved(x)
  setLen(x, y.len)
  var i = 0
  while i < y.len: `=copy`(x[i], y[i]); inc(i)

[zerbina]

This change will lead to a memory leak in the following case:

proc test() =
  var s = newSeq[int](1)
  let dst = addr s
  let src = addr s
  # `dst` and `src` point to the same location
  dst[] = src[]
  # expands to `=copy(dst[], src[])`

test()

The wasMoved(x) within the =copy will zero the payload pointer (while leaving the payload allocated), with the subsequent setLen then allocating a new payload and storing it in x.

Once control-flow leaves =copy, the temporary stack location still storing the old payload address stops (logically) existing and the old payload becomes unreachable -> memory leak.

[ringabout]

Make sense, thank you!