Create FIR fake source element for checked safe call subject

compiler/fir/raw-fir/raw-fir.common/src/org/jetbrains/kotlin/fir/builder/ConversionUtils.kt

@@ -499,7 +499,7 @@ fun FirQualifiedAccess.wrapWithSafeCall(receiver: FirExpression): FirSafeCallExp
         this.originalReceiverRef = FirExpressionRef<FirExpression>().apply {
             bind(receiver)
         }
-        this.source = receiver.source
+        this.source = receiver.source?.fakeElement(FirFakeSourceElementKind.CheckedSafeCallSubject)
     }
 
     replaceExplicitReceiver(checkedSafeCallSubject)

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirSourceElement.kt

@@ -154,6 +154,9 @@ sealed class FirFakeSourceElementKind : FirSourceElementKind() {
     // fun foo(vararg args: Int) {}
     // fun bar(1, 2, 3) --> [resolved] fun bar(VarargArgument(1, 2, 3))
     object VarargArgument : FirFakeSourceElementKind()
+
+    // Part of desugared x?.y
+    object CheckedSafeCallSubject : FirFakeSourceElementKind()
 }
 
 sealed class FirSourceElement {