[FIR] Fix local RETURN_TYPE_MISMATCH with flexible Unit and multiple Unit returns

compiler/testData/diagnostics/tests/functionLiterals/return/LocalReturnWithFlexibleUnitType.kt

@@ -0,0 +1,20 @@
+// FIR_IDENTICAL
+
+// FILE: JavaClass.java
+
+@FunctionalInterface
+public interface JavaClass<T> {
+    public T invoke();
+}
+
+// FILE: main.kt
+
+fun main() {
+    JavaClass {
+        if (true) {
+            return@JavaClass
+        } else {
+            return@JavaClass
+        }
+    }
+}

compiler/testData/diagnostics/tests/functionLiterals/return/MultipleLocalUnitReturns.kt

@@ -0,0 +1,11 @@
+// FIR_IDENTICAL
+
+fun <T> execute(block: () -> T): T = block()
+
+fun main() {
+    execute {
+        if (true) return@execute
+        if (true) return@execute Unit
+        execute { Any() }
+    }
+}