Add SymDenotation#isPublic

Also replace the `isPublic` in `DottyBackendInterface` by this one, note
that the new definition fixes a bug in the previous one:
private-qualified symbols do not have the `Private` flag set (see
`ParserCommon#normalize`) but are still not public.

Author: smarter

src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -569,7 +569,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile)(implicit ctx: Context
     def isConstructor: Boolean = toDenot(sym).isConstructor
     def isAnonymousFunction: Boolean = toDenot(sym).isAnonymousFunction
     def isMethod: Boolean = sym is Flags.Method
-    def isPublic: Boolean =  sym.flags.is(Flags.EmptyFlags, Flags.Private | Flags.Protected)
+    def isPublic: Boolean =  toDenot(sym).isPublic
     def isSynthetic: Boolean = sym is Flags.Synthetic
     def isPackageClass: Boolean = sym is Flags.PackageClass
     def isModuleClass: Boolean = sym is Flags.ModuleClass

src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1060,6 +1060,12 @@ object SymDenotations {
       else defn.RootClass
     }
 
+    /** Is this denotation public ? */
+    def isPublic(implicit ctx: Context): Boolean =
+      // private-qualified denotations do not have the Private flag set,
+      // see `ParserCommon#normalize`
+      !this.is(Protected | Private) && !privateWithin.exists
+
     /** The primary constructor of a class or trait, NoSymbol if not applicable. */
     def primaryConstructor(implicit ctx: Context): Symbol = NoSymbol