K2: Apply constraints from forks at UNTIL_FIRST_LAMBDA completion phase

Otherwise, OVERLOAD_RESOLUTION_AMBIGUITY is reported for the calls to
the functions annotated as @OverloadResolutionByLambdaReturnType

^KT-43296 In Progress

compiler/testData/diagnostics/tests/inference/inferenceForkRegressionSimple.kt

@@ -0,0 +1,21 @@
+// SKIP_TXT
+// FULL_JDK
+// WITH_STDLIB
+
+interface C : MutableMap<String, Int>
+
+fun <K, V> foo(m: MutableMap<K, V>, c: C) {
+    if (c === m) {
+        // `m` has a type C & MutableMap<K, V>
+        // So it has two instances of Map supertypes: Map<String, Int> and Map<K, V>
+        //
+        // Thus, it leads to constraint system fork during `flatMap` inference
+        // And during overload resolution between two `flatMap` versions with @OverloadResolutionByLambdaReturnType we have to run lambda analysis
+        // So, we have to apply forks to the system on this completion phase too, so we would have enough information for the input types of lambda
+        // Otherwise, OVERLOAD_RESOLUTION_AMBIGUITY happens
+        // But we don't do it for PARTIAL completion mode still
+        <!DEBUG_INFO_SMARTCAST!>c<!>.flatMap { _ ->
+            listOf("")
+        }
+    }
+}