Make isFromJava check work for SAM adapter extension descriptors (KT-21538)

Consider all callable descriptor in JavaClassDescriptor to be from Java.

This is used to check if smart step into should be intercepted by Kotlin
handler or delegated to Java.

 #KT-21538 Fixed

core/descriptors.jvm/src/org/jetbrains/kotlin/load/java/specialBuiltinMembers.kt

@@ -343,10 +343,10 @@ fun ClassDescriptor.hasRealKotlinSuperClassWithOverrideOf(
     return false
 }
 
-// Util methods
 val CallableMemberDescriptor.isFromJava: Boolean
-    get() = propertyIfAccessor.let { descriptor ->
-        (descriptor as? JavaCallableMemberDescriptor)?.containingDeclaration is JavaClassDescriptor
+    get() {
+        val descriptor = propertyIfAccessor
+        return descriptor.containingDeclaration is JavaClassDescriptor
     }
 
 fun CallableMemberDescriptor.isFromJavaOrBuiltins() = isFromJava || KotlinBuiltIns.isBuiltIn(this)