[FE 1.0] Don't lose diagnostics during lambda analysis at the overload resolution by return type stage

^KT-49658 Fixed

compiler/testData/diagnostics/tests/inference/kt49658Strict.fir.kt

@@ -0,0 +1,17 @@
+// !LANGUAGE: +DontLoseDiagnosticsDuringOverloadResolutionByReturnType
+// WITH_STDLIB
+
+fun doTheMapThing1(elements: List<CharSequence>): List<String> {
+    return elements.flatMap {
+        <!ARGUMENT_TYPE_MISMATCH!>when (it) { // NullPointerException
+            is String -> listOf("Yeah")
+            else -> null
+        }<!>
+    }
+}
+
+fun doTheMapThing2(elements: List<CharSequence>): List<String> {
+    return elements.flatMap {
+        <!ARGUMENT_TYPE_MISMATCH!>if (it is String) listOf("Yeah") else null<!> // it's OK with `if`
+    }
+}