[FIR] Set EnhancedNullability attribute for former flexible types

In some cases we transform flexible type into non-flexible during
  enhancement, but don't add `@EnhancedNullability` attribute to them,
  which breaks consistency with K1 on IR level

This commit fixes it

^KT-65302

compiler/testData/ir/irText/firProblems/putIfAbsent.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // TARGET_BACKEND: JVM
 // WITH_STDLIB
 // FULL_JDK