[FIR] Improved lambda completion: initial implementation

Repeat the logic of KotlinConstraintSystemCompleter in ConstraintSystemCompleter.
Implement additional context operations required for updated lambda completion algorithm.

compiler/testData/diagnostics/tests/inference/regressions/kt41394.fir.kt

@@ -0,0 +1,10 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_ANONYMOUS_PARAMETER
+// !LANGUAGE: +InferenceCompatibility
+
+fun <T, VR : T> foo(x: T, fn: (VR?, T) -> Unit) {}
+
+fun takeInt(x: Int) {}
+
+fun main(x: Int) {
+    foo(x) { prev: Int?, new -> <!INAPPLICABLE_CANDIDATE!>takeInt<!>(new) } // `new` is `Int` in OI, `Int?` in NI
+}

compiler/testData/diagnostics/tests/inference/regressions/kt41394.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_ANONYMOUS_PARAMETER
 // !LANGUAGE: +InferenceCompatibility