Polish and document separation checker (2)

Use a coarser prediction of hidden sets. Any Fresh.Cap instance
in a  capture set means that we should do the hidden check. Previously
this was the case only if the Fresh.Cap instance hid something non maximal,
element, but only directly, not via another Cap. Another version searched for
non-maximal references deeply, but this one caused infinite looping in the CI.
But in any case it's fine to over-approximate and assume that any Fresh.Cap
hides something.

Author: odersky

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -1839,7 +1839,7 @@ class CheckCaptures extends Recheck, SymTransformer:
       end checker
 
       checker.traverse(unit)(using ctx.withOwner(defn.RootClass))
-      if ccConfig.useSepChecks then SepChecker(this).traverse(unit)
+      if ccConfig.useSepChecks then SepCheck(this).traverse(unit)
       if !ctx.reporter.errorsReported then
         // We dont report errors here if previous errors were reported, because other
         // errors often result in bad applied types, but flagging these bad types gives

compiler/src/dotty/tools/dotc/cc/Fresh.scala

@@ -67,6 +67,7 @@ object Fresh:
       case _ => None
   end Cap
 
+  /** Map each occurrence of cap to a different Sep.Cap instance */
   class FromCap(owner: Symbol)(using Context) extends BiTypeMap, FollowAliasesMap:
     thisMap =>