Move cc annotations to non-bootstrapped src directory

Author: odersky

library/src-bootstrapped/scala/internal/requiresCapability.scala renamed to library/src/scala/annotation/internal/requiresCapability.scala

@@ -11,5 +11,6 @@ package scala.annotation
  *  The annotation can also be written explicitly if one wants to avoid the
  *  non-standard capturing type syntax.
  */
-@experimental class retains(xs: Any*) extends annotation.StaticAnnotation
+// @experimental // suppressed so we can use in compiler
+class retains(xs: Any*) extends annotation.StaticAnnotation