Also add privateWithin when creating constructor proxies

[odersky]

When creating a constructor proxy symbol we forgot to copy the privateWithin of the target class.

Fixes #18545

[odersky]

How I fixed it: Looking at the nice minimization I was first wondering where the apply came from, until I realized that it was a constructor proxy (i.e. a synthetic apply that gets converted into a new expression). This apply did not show up in the code after typer since it was expanded away. So the next step was to compile with -Yshow-tree-ids to find the node id of the New expression that was created and then with -Ydebug-tree-with-id to identify the code that created it. Sure enough that code (in Typer.newExpr) referred to the constructor proxy symbol. I added a println that displayed the symbol and compiled again with -uniqid. That gave me the id of the constructor proxy symbol. I then uncommented an assert in the Symbol constructor that was left there for precisely this reason:

//assert(id != 723)

became an uncommented assert that stopped when the symbol was created. Once I had the creation site (in NamerOps) it was easy to see that the privateWithin argument was not passed, so I added it.

[kubukoz]

Question: is this something that should be backported to 3.3 (LTS)?