Add scala2-library-cc-tasty project

[nicolasstucki]

We can override the sources of the standard library with cc annotation in this project and generate a TASTy version of the standard library.

Usage:

sbt> set ThisBuild/Build.scala2Library := Build.Scala2LibraryCCTasty
sbt> scala3-bootstrapped/testCompilation
sbt> scala3-bootstrapped/test
sbt> scala3-bootstrapped/run ...

sbt> scala2-library-cc/run diff
sbt> scala2-library-cc/run diff scala/Option.scala
sbt> scala2-library-cc/run diff scala/collection/immutable

Current limitations

Empty self types in library

We need to override Function1 and PartialFunction to remove their empty self type. These cause some problem with capture checking. We need to fix these cases. I will open an issue once we have a way to reproduce it with this PR.

Cannot use the TASTy jar yet

There is a bug when we try to class load the captured checked library from TASTy. These are due to some cyclic reference while unpickling the TASTy.

The current theory is that this due to the CC annotations in the top-level classes.
If this is the case, we can fix this issue by adding these language features as TASTy attributes (see #19033).
This was wrong or only part of the problems. The current issue can be reproduced with #19115.

[nicolasstucki]

The commit that adds a large number of files (64f3fdc) was created uisng

sbt> scala2-library-cc/run clone ...

where ... was the list of all sources that where cloned in that commit.

[sjrd]

We should reuse the sources of the CC stdlib instead of copy it once more.

[sjrd]

Suggested change

	jars.get(jarName) match {
	case Some(jar) =>
	Seq(s"-Ddotty.tests.tasties.scalaLibrary=$jar")
	case None =>
	log.warn("Scala 2 library TASTy is ignored on non-bootstrapped compiler")
	Seq.empty
	}
	jars.get(jarName) match {
	case Some(jar) =>
	Seq(s"-Ddotty.tests.tasties.scalaLibrary=$jar")
	case None =>
	log.warn("Scala 2 library TASTy is ignored on non-bootstrapped compiler")
	Seq.empty
	}