[NI] Approximate captured types before type variable fixation.

We prefer denotable types when we solve constraint system.
I.e. if for T we have not equality constraint with captured type we can approximate captured type to denotable type.

compiler/testData/diagnostics/tests/inference/capturedTypes/approximateBeforeFixation.kt

@@ -0,0 +1,8 @@
+
+fun <T> Array<out T>.intersect(other: Iterable<T>) {
+    val set = toMutableSet()
+    set.retainAll(other)
+}
+
+fun <X> Array<out X>.toMutableSet(): MutableSet<X> = TODO()
+fun <Y> MutableCollection<in Y>.retainAll(<!UNUSED_PARAMETER!>elements<!>: Iterable<Y>) {}

compiler/testData/diagnostics/tests/inference/capturedTypes/approximateBeforeFixation.txt

@@ -0,0 +1,5 @@
+package
+
+public fun </*0*/ T> kotlin.Array<out T>.intersect(/*0*/ other: kotlin.collections.Iterable<T>): kotlin.Unit
+public fun </*0*/ Y> kotlin.collections.MutableCollection<in Y>.retainAll(/*0*/ elements: kotlin.collections.Iterable<Y>): kotlin.Unit
+public fun </*0*/ X> kotlin.Array<out X>.toMutableSet(): kotlin.collections.MutableSet<X>