FIR: make declarations marked with 'override' implicitly open

#KT-52236 Fixed

compiler/fir/analysis-tests/testData/resolve/enum.fir.txt

@@ -26,7 +26,7 @@ FILE: enum.kt
                 super<R|SomeEnum|>(Q|O1|)
             }
 
-            public final override fun check(y: R|Some|): R|kotlin/Boolean| {
+            public open override fun check(y: R|Some|): R|kotlin/Boolean| {
                 ^check Boolean(true)
             }
 
@@ -37,7 +37,7 @@ FILE: enum.kt
                 super<R|SomeEnum|>(Q|O2|)
             }
 
-            public final override fun check(y: R|Some|): R|kotlin/Boolean| {
+            public open override fun check(y: R|Some|): R|kotlin/Boolean| {
                 ^check ==(R|<local>/y|, Q|O2|)
             }
 
@@ -81,11 +81,11 @@ FILE: enum.kt
                 super<R|EnumClass|>()
             }
 
-            public final override fun foo(): R|kotlin/Int| {
+            public open override fun foo(): R|kotlin/Int| {
                 ^foo Int(1)
             }
 
-            public final override val bar: R|kotlin/String| = String(a)
+            public open override val bar: R|kotlin/String| = String(a)
                 public get(): R|kotlin/String|
 
         }