Wrong diagnostic when last expression in lambda contains assignment operation

#KT-6769 Fixed

compiler/testData/diagnostics/tests/functionLiterals/kt6869.kt

@@ -0,0 +1,11 @@
+fun main(args : Array<String>) {
+    var list = listOf(1)
+
+    val a: Int? = 2
+
+    a?.let { list += it }
+}
+
+fun <T : Any, R> T.let(f: (T) -> R): R = f(this)
+fun <T> Iterable<T>.plus(<!UNUSED_PARAMETER!>element<!>: T): List<T> = null!!
+fun listOf<T>(vararg <!UNUSED_PARAMETER!>values<!>: T): List<T> = null!!