Potentially confusing behavior of `dbg` in main

[sabiwara]

Elixir and Erlang/OTP versions

Main branch

Operating system

n/a

Current behavior

Since we changed dbg to print intermediate results addressing #14005, the header and value can end up completely separated when the evaluated expression emits its own nested dbg:

defmodule Baz do
  def foo(x), do: dbg(x)
  def bar(x), do: foo(x) |> dbg()
end

iex> A.bar(1)
[iex:1: A.bar/1]
[iex:1: A.foo/1]
x #=> 1

foo(x) #=> 1

Expected behavior

In 1.18:

iex> A.bar(1)
[iex:1: A.foo/1]
x #=> 1

[iex:1: A.bar/1]
foo(x) #=> 1

The new behavior is arguably confusing, but I'm not sure if this can be fixed without reverting to the old approach - this might be a trade-off?

[josevalim]

I pushed a fix so we print the header with the first output, it improves this case, but we may still have mixed output in other scenarios (if/block/etc) but those are likely fine as they have the content attached to them too.