FIR: handle synthetic properties with unstable smartcast

Synthetic properties from Java getter/setters need to be specially
handled so that candidates from such symbols are marked with unstable.

compiler/testData/diagnostics/tests/smartCasts/inference/syntheticPropertyOnUnstableSmartcast.kt

@@ -0,0 +1,29 @@
+// FIR_IDENTICAL
+// FILE: p/Super.java
+
+package p
+
+public interface Super {
+    public String getName()
+    public void setName(String name)
+}
+
+// FILE: p/test.kt
+
+package p
+
+class Sub : Super {
+    val onlyInSub: Int = 1
+    override fun getName(): String = ""
+    override fun setName(name: String) {}
+}
+
+var s: Super = Sub()
+
+fun test() {
+    if (s is Sub) {
+        s.name
+        s.name = ""
+        <!SMARTCAST_IMPOSSIBLE!>s<!>.onlyInSub
+    }
+}