Apply flexible types to files compiled without explicit nulls

[HarrisL2]

A fixed version of #22473

This PR wraps the types of symbols from files compiled without explicit nulls in flexible types.
This allows for interop between multiple files in cases where

class Unsafe_1 {
  def foo(s: String): String = {
    if (s == null) then "nullString"
    else s
  }
}

compiled without explicit nulls can still be used in

def Flexible_2() =
  val s2: String | Null = "foo"
  val unsafe = new Unsafe_1()
  val s: String = unsafe.foo(s2)
  unsafe.foo("")
  unsafe.foo(null)

whereas the argument would have been a strictly non-null String, because of the flexible type, the function call is now permitted.

[olhotak]

Test performance please

[olhotak]

[test_scala2_library_tasty]

[noti0na1]

test performance please

[olhotak]

Please add a commit that doesn't change anything real to force the CI to re-run. (Perhaps change the newly-introduced double newline in TreeUnpickler.)

Otherwise LGTM, but also wait for approval from @noti0na1 .