FIR: make declarations marked with 'override' implicitly open

#KT-52236 Fixed

compiler/fir/analysis-tests/testData/resolve/fromBuilder/enums.fir.txt

@@ -33,7 +33,7 @@ FILE: enums.kt
                 super<R|Planet|>(Double(1.0), Double(2.0))
             }
 
-            public final override fun sayHello(): R|kotlin/Unit| {
+            public open override fun sayHello(): R|kotlin/Unit| {
                 <Unresolved name: println>#(String(Hello!!!))
             }
 
@@ -44,7 +44,7 @@ FILE: enums.kt
                 super<R|Planet|>(Double(3.0), Double(4.0))
             }
 
-            public final override fun sayHello(): R|kotlin/Unit| {
+            public open override fun sayHello(): R|kotlin/Unit| {
                 <Unresolved name: println>#(String(Ola!!!))
             }
 
@@ -55,7 +55,7 @@ FILE: enums.kt
                 super<R|Planet|>(Double(5.0), Double(6.0))
             }
 
-            public final override fun sayHello(): R|kotlin/Unit| {
+            public open override fun sayHello(): R|kotlin/Unit| {
                 <Unresolved name: println>#(String(Privet!!!))
             }