FIR: make declarations marked with 'override' implicitly open

#KT-52236 Fixed

compiler/fir/analysis-tests/testData/resolveWithStdlib/intellij/EnumWithToString.fir.txt

@@ -9,7 +9,7 @@ FILE: EnumWithToString.kt
                 super<R|Some|>()
             }
 
-            public final override fun toString(): R|kotlin/String| {
+            public open override fun toString(): R|kotlin/String| {
                 ^toString String(Entry)
             }
 

compiler/fir/analysis-tests/testData/resolveWithStdlib/intellij/baseIntrospector.fir.txt

@@ -54,7 +54,7 @@ FILE: m2.kt
             super<R|m1/BaseSingleDatabaseIntrospector<m2/SqliteRoot, m2/SqliteSchema>|>()
         }
 
-        protected final override fun createDatabaseRetriever(database: R|m2/SqliteRoot|): R|m1/BaseIntrospector.AbstractDatabaseRetriever<m2/SqliteRoot, m2/SqliteRoot, m2/SqliteSchema>| {
+        protected open override fun createDatabaseRetriever(database: R|m2/SqliteRoot|): R|m1/BaseIntrospector.AbstractDatabaseRetriever<m2/SqliteRoot, m2/SqliteRoot, m2/SqliteSchema>| {
             ^createDatabaseRetriever object : R|m1/BaseIntrospector.AbstractDatabaseRetriever<m2/SqliteRoot, m2/SqliteRoot, m2/SqliteSchema>| {
                 private constructor(): R|<anonymous>| {
                     this@R|m2/SqliteIntrospector|.super<R|m1/BaseIntrospector.AbstractDatabaseRetriever<m2/SqliteRoot, m2/SqliteRoot, m2/SqliteSchema>|>(R|<local>/database|)

compiler/fir/analysis-tests/testData/resolveWithStdlib/intellij/defaultParameterFromBase.fir.txt

@@ -13,7 +13,7 @@ FILE: defaultParameterFromBase.kt
             super<R|Base|>()
         }
 
-        public final override fun foo(arg: R|kotlin/Int|, def: R|kotlin/String|): R|kotlin/Unit| {
+        public open override fun foo(arg: R|kotlin/Int|, def: R|kotlin/String|): R|kotlin/Unit| {
         }
 
     }