Fix soundness hole of forgotten reach capabilities

[odersky]

Fixes #20503

[Linyxus]

LGTM!

[hamzaremmal]

The following test failed in nightly with Scala 2 libary CC TASTy.

Wrong number of errors encountered when compiling tests/neg/i20503.scala
expected: 1, actual: 2
Unfulfilled expectations:

Unexpected errors:
tests/neg/i20503.scala:3
-> following the errors:
 at 3: Reach capability cap and universal capability cap cannot both
appear in the type [U](f: box (() ->{ops*} Unit) => U): Unit of this expression
 at 6: Found:    (ops: List[box () ->? Unit]^?) => Unit
Required: (ops: List[box () => Unit]) -> Unit

See: https://github.com/scala/scala3/actions/runs/9377502987/job/25819168381

[hamzaremmal]

Fixed by #20528

[odersky]

@noti0na1 @Linyxus Can one of you follow up on this please. I don't have the bandwidth right now.

[noti0na1]

I will look into this error

[odersky]

@noti0na1 Thank you!

[noti0na1]

The extra error is at the type of foreach: [U](f: A => U): Unit.

When compiled with Scala2LibraryCCTasty using the bootstrapped compiler, symbol foreach is considered already checked with CC, so it is not fluidified.

I tried the commit before this PR, and I can trigger the same error using the same setting. So this error is not related to this PR.