Add platform/impl soft keywords, add isPlatform/isImpl to MemberDescriptor

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/descriptors/KnownDescriptors.kt

@@ -138,6 +138,8 @@ open class KnownClassDescriptor(
     override fun isCompanionObject(): Boolean = false
     override fun isData(): Boolean = false
     override fun isInner(): Boolean = false
+    override fun isPlatform(): Boolean = false
+    override fun isImpl(): Boolean = false
 
     override fun <R : Any?, D : Any?> accept(visitor: DeclarationDescriptorVisitor<R, D>, data: D): R {
         return visitor.visitClassDescriptor(this, data)