Fix #15363: Improve error messages for leaking of this

[liufengyun]

Fix #15363: Improve error messages for leaking of this

class A:
  // should report one error here
  val b = new B(this) // error
  val m = 10
  val n = 20

class B(a: A):
  val x = a.m
  val y = a.n

For the code above, we now report the following error:

-- Error: tests/init/neg/i15363.scala:3:10 -----------------------------------------------------------------------------
3 |  val b = new B(this) // error
  |          ^^^^^^^^^^^
  |          Problematic object instantiation: arg 1 is not fully initialized. Calling trace:
  |          -> class A:	[ i15363.scala:1 ]
  |             ^
  |          -> val b = new B(this) // error	[ i15363.scala:3 ]
  |                     ^^^^^^^^^^^
  |
  |          It leads to the following error during object initialization:
  |          Access field on a value with an unknown initialization status. Calling trace:
  |          -> class B(a: A):	[ i15363.scala:7 ]
  |             ^
  |          -> val x = a.m	[ i15363.scala:8 ]
  |                     ^^^

[olhotak]

Overall a big improvement.

[olhotak]

Is it guaranteed that all errors are uses of one of the passed non-hot values? Could there be an error in the called constructor that is unrelated to the passed arguments?

[olhotak]

It would be good to print the class name of the outer in the error message.

[olhotak]

Otherwise LGTM pending a fix for #15374.

[liufengyun]

@olhotak I created #15465 to track the issue so that we can merge this one to unblock other PRs.