[backport] Fix superclass for Java interface symbols created in JavaMirrors

According to the spec [1] the superclass of an interface is always
Object.

Restores the tests that were moved to pending in bf951ec,
fixex part of SI-9374.

[1] https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.1

Author: lrytz

src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala

@@ -284,7 +284,7 @@ abstract class ClassfileParser {
 
     def getType(index: Int): Type              = getType(null, index)
     def getType(sym: Symbol, index: Int): Type = sigToType(sym, getExternalName(index))
-    def getSuperClass(index: Int): Symbol      = if (index == 0) AnyClass else getClassSymbol(index)
+    def getSuperClass(index: Int): Symbol      = if (index == 0) AnyClass else getClassSymbol(index) // the only classfile that is allowed to have `0` in the super_class is java/lang/Object (see jvm spec)
 
     private def createConstant(index: Int): Constant = {
       val start = starts(index)

src/reflect/scala/reflect/runtime/JavaMirrors.scala

@@ -755,6 +755,7 @@ private[scala] trait JavaMirrors extends internal.SymbolTable with api.JavaUnive
           val ifaces = jclazz.getGenericInterfaces.toList map typeToScala
           val isAnnotation = JavaAccFlags(jclazz).isAnnotation
           if (isAnnotation) AnnotationClass.tpe :: ClassfileAnnotationClass.tpe :: ifaces
+          else if (jclazz.isInterface) ObjectTpe :: ifaces // interfaces have Object as superclass in the classfile (see jvm spec), but getGenericSuperclass seems to return null
           else (if (jsuperclazz == null) AnyTpe else typeToScala(jsuperclazz)) :: ifaces
         } finally {
           parentsLevel -= 1