[FIR] Resolve receiver in qualified expressions with no selector

In qualified expression like `foo().`, selector expression is null.
Because of that the whole expression was marked as an error FIR
expression, and `foo()` part was not resolved at all (including
arguments and everything else).

This commit fixes the problem by providing receiver's FIR expression
as an underlying expression for error FIR expression. That way
it will be seen by all resolve transformers and will be successfully
resolved.

^KTIJ-21484 Fixed

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/ExpressionsConverter.kt

@@ -554,10 +554,13 @@ class ExpressionsConverter(
             result = convertFirSelector(it, dotQualifiedExpression.toFirSourceElement(), firReceiver!!) as? FirExpression
         }
 
-        return result ?: buildErrorExpression(
-            null,
-            ConeSimpleDiagnostic("Qualified expression without selector", DiagnosticKind.Syntax)
-        )
+        return result ?: buildErrorExpression {
+            source = null
+            diagnostic = ConeSimpleDiagnostic("Qualified expression without selector", DiagnosticKind.Syntax)
+
+            // if there is no selector, we still want to resolve the receiver
+            expression = firReceiver
+        }
     }
 
     /**

compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt

@@ -2389,14 +2389,19 @@ open class RawFirBuilder(
         }
 
         override fun visitQualifiedExpression(expression: KtQualifiedExpression, data: Unit): FirElement {
+            val receiver = expression.receiverExpression.toFirExpression("Incorrect receiver expression")
+
             val selector = expression.selectorExpression
-                ?: return buildErrorExpression(
-                    expression.toFirSourceElement(), ConeSimpleDiagnostic("Qualified expression without selector", DiagnosticKind.Syntax),
-                )
+                ?: return buildErrorExpression {
+                    source = expression.toFirSourceElement()
+                    diagnostic = ConeSimpleDiagnostic("Qualified expression without selector", DiagnosticKind.Syntax)
+
+                    // if there is no selector, we still want to resolve the receiver
+                    this.expression = receiver
+                }
+
             val firSelector = selector.toFirExpression("Incorrect selector expression")
             if (firSelector is FirQualifiedAccess) {
-                val receiver = expression.receiverExpression.toFirExpression("Incorrect receiver expression")
-
                 if (expression is KtSafeQualifiedExpression) {
                     @OptIn(FirImplementationDetail::class)
                     firSelector.replaceSource(expression.toFirSourceElement(KtFakeSourceElementKind.DesugaredSafeCallExpression))