[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/fir/analysis-tests/testData/resolveWithStdlib/problems/EnumMapGet.fir.txt

@@ -1,5 +1,5 @@
 FILE: EnumMapGet.kt
     public final typealias SomeEnumMap = R|java/util/EnumMap<kotlin/String, kotlin/String?>|
     public final fun test(map: R|SomeEnumMap|, qualifier: R|kotlin/String?|): R|kotlin/Unit| {
-        lval result: R|kotlin/String?| = R|<local>/map|.R|SubstitutionOverride<java/util/EnumMap.get: R|kotlin/String?|>|(R|<local>/qualifier|)
+        lval result: R|kotlin/String?| = R|<local>/map|.R|SubstitutionOverride<java/util/EnumMap.get: R|@EnhancedNullability kotlin/String?|>|(R|<local>/qualifier|)
     }

compiler/fir/analysis-tests/testData/resolveWithStdlib/problems/NamedParameterInConcurrentMap.fir.txt

@@ -1,5 +1,5 @@
 FILE: NamedParameterInConcurrentMap.kt
     public final fun main(): R|kotlin/Unit| {
         lval map: R|java/util/concurrent/ConcurrentHashMap<kotlin/String, kotlin/String>| = R|java/util/concurrent/ConcurrentHashMap.ConcurrentHashMap|<R|kotlin/String|, R|kotlin/String|>()
-        R|<local>/map|.R|SubstitutionOverride<java/util/concurrent/ConcurrentHashMap.put: R|kotlin/String?|>|(key = String(key), value = String(value))
+        R|<local>/map|.R|SubstitutionOverride<java/util/concurrent/ConcurrentHashMap.put: R|@EnhancedNullability kotlin/String?|>|(key = String(key), value = String(value))
     }

compiler/fir/analysis-tests/testData/resolveWithStdlib/problems/weakHashMap.fir.txt

@@ -2,5 +2,5 @@ FILE: weakHashMap.kt
     public final val someMap: R|java/util/WeakHashMap<kotlin/Any?, kotlin/Any?>| = R|java/util/WeakHashMap.WeakHashMap|<R|kotlin/Any?|, R|kotlin/Any?|>()
         public get(): R|java/util/WeakHashMap<kotlin/Any?, kotlin/Any?>|
     public final fun foo(): R|kotlin/Unit| {
-        R|/someMap|.R|SubstitutionOverride<java/util/WeakHashMap.get: R|kotlin/Any?|>|(String())
+        R|/someMap|.R|SubstitutionOverride<java/util/WeakHashMap.get: R|@EnhancedNullability kotlin/Any?|>|(String())
     }