Capture Checking of Lazy Vals

[bracevac]

Fixes #21601

Treat lazy vals similarly to parameterless methods.

TODOs

• Basic lazy vals support under vanilla CC
  • More spec tests
• Support for lazy vals under separation checking (mental model: transparent ref cell + pure/rdonly init function)

[bracevac]

Under this scheme, private lazy val _sorted: Seq[A] in scala/collection/SeqView.scala will (rightfully) capture var underlying : SomeSeqOps[A]^, which in turn will blow up on this line

override def to[C1](factory: Factory[A, C1]): C1 = _sorted.to(factory)

Because we expect to return a pure C1, whereas in fact it isn't now.

(I'm glad that this appears to be the only place in the library that causes trouble).

[bracevac]

After some more thought, it's actually quite puzzling that the _sorted.to(factory) call would be impure. We'd expect the result to be pure, and only charge the _sorted initializer's capture set to the context.

I've now reverted to the previous solution with just includeCallCaptures. This version does not support treating a lazy val x: Foo = ... with impure initializer as a trackable resource. E.g., () => x is not of type () ->{x} Foo, but () ->{<init>} Foo, and we can't say that {x} <: {<init>}.