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
+    }
+}

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

@@ -0,0 +1,25 @@
+package
+
+package p {
+    public var s: p.Super
+    public fun test(): kotlin.Unit
+
+    public final class Sub : p.Super {
+        public constructor Sub()
+        public final val onlyInSub: kotlin.Int = 1
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ fun getName(): kotlin.String
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ fun setName(/*0*/ name: kotlin.String): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public interface Super {
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public abstract fun getName(): kotlin.String!
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public abstract fun setName(/*0*/ name: kotlin.String!): kotlin.Unit
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}
+