[FIR] Make FirDeclaration an abstract class

compiler/fir/jvm/src/org/jetbrains/kotlin/fir/resolve/calls/jvm/ConeEquivalentCallConflictResolver.kt

@@ -68,7 +68,7 @@ class ConeEquivalentCallConflictResolver(
         return when (declaration) {
             is FirSimpleFunction -> createFlatSignature(call, declaration)
             is FirConstructor -> createFlatSignature(call, declaration)
-            is FirVariable<*> -> createFlatSignature(call, declaration as FirVariable<*>)
+            is FirVariable<*> -> createFlatSignature(call, declaration)
             else -> error("Not supported: $declaration")
         }
     }

compiler/fir/jvm/src/org/jetbrains/kotlin/fir/scopes/jvm/DescriptorUtils.kt

@@ -21,7 +21,6 @@ import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
 
 fun FirFunction<*>.computeJvmSignature(typeConversion: (FirTypeRef) -> ConeKotlinType? = FirTypeRef::coneTypeSafe): String? {
-    if (this !is FirCallableMemberDeclaration<*>) return null
     val containingClass = containingClass() ?: return null
 
     return SignatureBuildingComponents.signature(containingClass.classId, computeJvmDescriptor(typeConversion = typeConversion))