FIR: Refine inference constraints when type variable in flexible position

That issue might be fixed via changing
TypeVariableMarker.shouldBeFlexible at ConeConstraintSystemUtilContext
but this and some other tricks have been added because of incorrect
handling of constraints where type variable has a flexible bound

^KT-51168 Fixed

compiler/testData/diagnostics/testsWithStdLib/purelyImplementedCollection/arrayListNullable.fir.kt

@@ -14,7 +14,7 @@ fun foo() {
     x[0] = ""
 
     val b1: MutableList<String?> = x
-    val b2: MutableList<String> = x
+    val b2: MutableList<String> = <!INITIALIZER_TYPE_MISMATCH!>x<!>
     val b3: List<String?> = x
 
     val b4: Collection<String?> = x