Make more anonymous functions static

An anonymous function in a static object was previously mapped to a member
of that object. We now map it to a static member of the toplevel class instead.
This causes the backend to memoize the function, which fixes scala#19224. On the
other hand, we don't do that for anonymous functions nested in the object
constructor, since that can cause deadlocks (see run/deadlock.scala).

Scala 2's behavior is different: it does lift lambdas in constructors
to be static, too, which can cause deadlocks.

Fixes scala#19924

Author: odersky

tests/run/i19224.scala

@@ -4,8 +4,6 @@ object Test extends App:
   def x(): Int => String = (i: Int) => i.toString
   def y(): () => String = () => field.toString
   locally:
-    assert(x() == x()) // was true on Scala 2, false on Scala 3...
+    assert(x() == x()) // true on Scala 2, was false on Scala 3...
     assert(y() == y()) // also true if `y` accesses object-local fields
 
-
-//@main def Test = App()