[FIR] Fix capturing of flexible types during resolution

Previously, because we didn't handle flexible types properly in
prepareCapturedType, projections inside flexible types would only be
captured during subtyping with captureStatus=FOR_SUBTYPING
which would lead to the constraint type being wrongly approximated
(see ConstraintInjector.TypeCheckerStateForConstraintInjector
.addNewIncorporatedConstraint).

Fixing the capturing produced two kinds of false positive diagnostics:

1. In ConstraintInjector.TypeCheckerStateForConstraintInjector
.addNewIncorporatedConstraint we would get two instances of cone types
that are structurally equal and containing the same captured type.
However, because we only skipped subtyping if the types were
referentially equal, we would get a contradiction here.
The fix was to use structural equality instead, which should be okay
as the captured type instances are the same.

2. Reified type variables were inferred to captured types because
flexible arrays with captured upper bounds
(Array<Foo>..Array<Captured(out Foo)>?) were not properly approximated.

#KT-62609 Fixed

compiler/testData/diagnostics/tests/inference/commonSystem/nonFixedVariableFromBothBranches.kt

@@ -12,8 +12,16 @@ fun <K> select(x: K, y: K): K = x
 
 fun <R> foo(f: () -> R): R = f()
 
+interface Inv<T> {
+    fun createArray(): Array<T>
+}
+
 fun test(n: Number) {
     val a = select(foo { JavaTest.createNumberArray() }, emptyArray())
-
     <!DEBUG_INFO_EXPRESSION_TYPE("(kotlin.Array<(kotlin.Number..kotlin.Number?)>..kotlin.Array<out (kotlin.Number..kotlin.Number?)>?)")!>a<!>
-}
+}
+
+fun test2(inv: Inv<out CharSequence>) {
+    val a = select(foo { inv.createArray() }, emptyArray())
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Array<out kotlin.CharSequence>")!>a<!>
+}