[FIR] Add flexible default upper bound for java type parameters

compiler/fir/analysis-tests/testData/enhancement/signatureAnnotations/DefaultNull.fir.txt

@@ -6,9 +6,9 @@ public open class A : R|kotlin/Any| {
     public constructor(): R|A|
 
 }
-public open class B<T> : R|kotlin/Any| {
+public open class B<T : R|ft<kotlin/Any, kotlin/Any?>!|> : R|kotlin/Any| {
     public open fun foo(@R|kotlin/annotations/jvm/internal/DefaultNull|() t: R|ft<T, T?>!| = Null(null)): R|kotlin/Unit|
 
-    public constructor<T>(): R|B<T>|
+    public constructor<T : R|ft<kotlin/Any, kotlin/Any?>!|>(): R|B<T>|
 
 }

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/JavaSymbolProvider.kt

@@ -113,7 +113,7 @@ class JavaSymbolProvider(
                 forTypeParameterBounds = true
             )
         }
-        addDefaultBoundIfNecessary()
+        addDefaultBoundIfNecessary(isFlexible = true)
     }
 
     private fun List<JavaTypeParameter>.convertTypeParameters(stack: JavaTypeParameterStack): List<FirTypeParameter> {

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirDeclarationUtil.kt

@@ -12,11 +12,21 @@ import org.jetbrains.kotlin.fir.symbols.impl.FirAnonymousObjectSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
 import org.jetbrains.kotlin.fir.types.ConeClassLikeType
+import org.jetbrains.kotlin.fir.types.ConeFlexibleType
+import org.jetbrains.kotlin.fir.types.builder.buildResolvedTypeRef
 import org.jetbrains.kotlin.fir.types.coneTypeSafe
 
-fun FirTypeParameterBuilder.addDefaultBoundIfNecessary() {
+fun FirTypeParameterBuilder.addDefaultBoundIfNecessary(isFlexible: Boolean = false) {
     if (bounds.isEmpty()) {
-        bounds += session.builtinTypes.nullableAnyType
+        val type = if (isFlexible) {
+            val session = this.session
+            buildResolvedTypeRef {
+                type = ConeFlexibleType(session.builtinTypes.anyType.type, session.builtinTypes.nullableAnyType.type)
+            }
+        } else {
+            session.builtinTypes.nullableAnyType
+        }
+        bounds += type
     }
 }