[FIR] Add proper constraint if Unit upper bound leads to Unit lambda return type

#KT-63524 Fixed

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirLightTreeOldFrontendDiagnosticsTestGenerated.java

@@ -17052,6 +17052,12 @@ public class FirLightTreeOldFrontendDiagnosticsTestGenerated extends AbstractFir
                 runTest("compiler/testData/diagnostics/tests/inference/underscoredTypeInForbiddenPositions.kt");
             }
 
+            @Test
+            @TestMetadata("UnitUpperboundLambdaReturnType.kt")
+            public void testUnitUpperboundLambdaReturnType() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/UnitUpperboundLambdaReturnType.kt");
+            }
+
             @Test
             @TestMetadata("unsoundness1.kt")
             public void testUnsoundness1() throws Exception {

compiler/fir/analysis-tests/tests-gen/org/jetbrains/kotlin/test/runners/FirPsiOldFrontendDiagnosticsTestGenerated.java

@@ -17058,6 +17058,12 @@ public class FirPsiOldFrontendDiagnosticsTestGenerated extends AbstractFirPsiDia
                 runTest("compiler/testData/diagnostics/tests/inference/underscoredTypeInForbiddenPositions.kt");
             }
 
+            @Test
+            @TestMetadata("UnitUpperboundLambdaReturnType.kt")
+            public void testUnitUpperboundLambdaReturnType() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/inference/UnitUpperboundLambdaReturnType.kt");
+            }
+
             @Test
             @TestMetadata("unsoundness1.kt")
             public void testUnsoundness1() throws Exception {

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/PostponedArgumentsAnalyzer.kt

@@ -201,10 +201,10 @@ class PostponedArgumentsAnalyzer(
             }
         }
 
-        if (!hasExpressionInReturnArguments && !lambdaExpectedTypeIsUnit) {
+        if (!hasExpressionInReturnArguments) {
             builder.addSubtypeConstraint(
                 components.session.builtinTypes.unitType.type,
-                returnTypeRef.type,
+                substitute(lambda.returnType),
                 ConeLambdaArgumentConstraintPosition(lambda.atom)
             )
         }