Psi2ir: do not generate default accessor body for expect properties

Because generateDefaultGetterBody/generateDefaultSetterBody reference
the property's backing field, which in case of extension properties
leads to an error "Unbound symbols not allowed" because extension
property cannot have a backing field.

In some way, this check is similar to the `isExpect` check in
`generatePrimaryConstructor`.

compiler/testData/codegen/box/multiplatform/expectProperty.kt

@@ -0,0 +1,26 @@
+// !LANGUAGE: +MultiPlatformProjects
+// FILE: common.kt
+
+package test
+
+expect val v: String
+
+expect val Char.extensionVal: String
+
+expect var String.extensionVar: Char
+
+// FILE: jvm.kt
+
+package test
+
+actual val v: String = ""
+
+actual val Char.extensionVal: String
+    get() = toString()
+
+actual var String.extensionVar: Char
+    get() = this[0]
+    set(value) {}
+
+fun box(): String =
+    v + 'O'.extensionVal + "K".extensionVar