Support KParameter.isOptional

#KT-8825 Fixed

compiler/testData/codegen/boxWithStdlib/reflection/parameters/isOptional.kt

@@ -0,0 +1,24 @@
+import kotlin.test.*
+
+open class A {
+    open fun foo(x: Int, y: Int = 1) {}
+}
+
+class B : A() {
+    override fun foo(x: Int, y: Int) {}
+}
+
+class C : A()
+
+
+fun Int.extFun() {}
+
+fun box(): String {
+    assertEquals(listOf(false, false, true), A::foo.parameters.map { it.isOptional })
+    assertEquals(listOf(false, false, true), B::foo.parameters.map { it.isOptional })
+    assertEquals(listOf(false, false, true), C::foo.parameters.map { it.isOptional })
+
+    assertFalse(Int::extFun.parameters.single().isOptional)
+
+    return "OK"
+}