NI: allow lower bound of flexible type for coercion-to-Unit

Example from
box/inference/coercionToUnitForLambdaReturnTypeWithFlexibleConstraint

// FILE: TestJ.java
public class TestJ {
  public static <T> In<T> materialize() { return null; }
}

// FILE: test.kt

class In<in T>

fun <T> inferred(e: In<T>?, l: () -> T): T = l()

fun box() {
  inferred(TestJ.materialize<Unit>(), { null })
}

`materialize` has flexible type, both for `In<T>` and `T`.
When analyzing `{ null }`, collected type constraints include:
ft<Unit?, Unit> <: T (from ft<In<ft<Unit, Unit?>>, In<ft<Unit, Unit?>>?>)

By allowing the lower bound of flexible type, FIR resolution can visit
`{ null }` with the expected type Unit, which will lead to proper
coercion to Unit at the end.

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/model/NewConstraintSystemImpl.kt

@@ -437,6 +437,9 @@ class NewConstraintSystemImpl(
     override fun hasUpperOrEqualUnitConstraint(type: KotlinTypeMarker): Boolean {
         checkState(State.BUILDING, State.COMPLETION, State.FREEZED)
         val constraints = storage.notFixedTypeVariables[type.typeConstructor()]?.constraints ?: return false
-        return constraints.any { (it.kind == ConstraintKind.UPPER || it.kind == ConstraintKind.EQUALITY) && it.type.isUnit() }
+        return constraints.any {
+            (it.kind == ConstraintKind.UPPER || it.kind == ConstraintKind.EQUALITY) &&
+                    it.type.lowerBoundIfFlexible().isUnit()
+        }
     }
 }