[FIR] Add opt-in annotation which prevents from using symbol.fir

compiler/fir/tree/src/org/jetbrains/kotlin/fir/symbols/FirBasedSymbol.kt

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.fir.declarations.FirDeclarationOrigin
 abstract class FirBasedSymbol<E : FirDeclaration> {
     private var _fir: E? = null
 
+    @SymbolInternals
     val fir: E
         get() = _fir
             ?: error("Fir is not initialized for $this")
@@ -30,3 +31,6 @@ abstract class FirBasedSymbol<E : FirDeclaration> {
     val moduleData: FirModuleData
         get() = fir.moduleData
 }
+
+@RequiresOptIn
+annotation class SymbolInternals