[FIR] Fix enhancement of FlexibleNullability and EnhancedNullability

- Add utilities to add new attribute to ConeAttributes
- Get rid of FlexibleNullability attribute (it can be easily inferred
    for any flexible type at any moment)
- Fix determining of EnhancedNullability attribute

compiler/testData/loadJava/compiledJava/javaBean/DifferentGetterAndSetter.fir.txt

@@ -1,5 +1,5 @@
 public open class DifferentGetterAndSetter : R|kotlin/Any| {
-    public open fun setSomething(color: R|ft<@FlexibleNullability kotlin/String, kotlin/String?>!|): R|kotlin/Unit|
+    public open fun setSomething(color: R|ft<kotlin/String, kotlin/String?>!|): R|kotlin/Unit|
 
     public open fun getSomething(): R|kotlin/Int|
 

compiler/testData/loadJava/compiledJava/javaBean/JavaBeanVal.fir.txt

@@ -1,5 +1,5 @@
 public open class JavaBeanVal : R|kotlin/Any| {
-    public open fun getColor(): R|ft<@FlexibleNullability kotlin/String, kotlin/String?>!|
+    public open fun getColor(): R|ft<kotlin/String, kotlin/String?>!|
 
     public constructor(): R|test/JavaBeanVal|
 

compiler/testData/loadJava/compiledJava/javaBean/JavaBeanVar.fir.txt

@@ -1,7 +1,7 @@
 public open class JavaBeanVar : R|kotlin/Any| {
-    public open fun getColor(): R|ft<@FlexibleNullability kotlin/String, kotlin/String?>!|
+    public open fun getColor(): R|ft<kotlin/String, kotlin/String?>!|
 
-    public open fun setColor(string: R|ft<@FlexibleNullability kotlin/String, kotlin/String?>!|): R|kotlin/Unit|
+    public open fun setColor(string: R|ft<kotlin/String, kotlin/String?>!|): R|kotlin/Unit|
 
     public constructor(): R|test/JavaBeanVar|
 

compiler/testData/loadJava/compiledJava/javaBean/JavaBeanVarOfGenericType.fir.txt

@@ -1,7 +1,7 @@
 public open class JavaBeanVarOfGenericType<P : R|ft<kotlin/Any, kotlin/Any?>!|> : R|kotlin/Any| {
-    public open fun getCharacters(): R|ft<@FlexibleNullability java/util/ArrayList<ft<@FlexibleNullability P, P?>!>, java/util/ArrayList<ft<@FlexibleNullability P, P?>!>?>!|
+    public open fun getCharacters(): R|ft<java/util/ArrayList<ft<P, P?>!>, java/util/ArrayList<ft<P, P?>!>?>!|
 
-    public open fun setCharacters(characters: R|ft<@FlexibleNullability java/util/ArrayList<ft<@FlexibleNullability P, P?>!>, java/util/ArrayList<ft<@FlexibleNullability P, P?>!>?>!|): R|kotlin/Unit|
+    public open fun setCharacters(characters: R|ft<java/util/ArrayList<ft<P, P?>!>, java/util/ArrayList<ft<P, P?>!>?>!|): R|kotlin/Unit|
 
     public constructor<P : R|ft<kotlin/Any, kotlin/Any?>!|>(): R|test/JavaBeanVarOfGenericType<P>|
 

compiler/testData/loadJava/compiledJava/javaBean/TwoSetters.fir.txt

@@ -1,5 +1,5 @@
 public open class TwoSetters : R|kotlin/Any| {
-    public open fun setSize(size: R|ft<@FlexibleNullability kotlin/String, kotlin/String?>!|): R|kotlin/Unit|
+    public open fun setSize(size: R|ft<kotlin/String, kotlin/String?>!|): R|kotlin/Unit|
 
     public open fun setSize(size: R|kotlin/Int|): R|kotlin/Unit|