[FIR2IR] Allow symbolTable cache lookups if IR f/o builder mode is enabled

Without this change, the following test is failing:
- FirLightTreeBlackBoxCodegenWithIrFakeOverrideGeneratorTestGenerated.Collections.testInheritFromHashtable

^KT-58861

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrDeclarationStorage.kt

@@ -1018,15 +1018,29 @@ class Fir2IrDeclarationStorage(
          */
         signatureCalculator()?.let { signature ->
             val cachedInSymbolTable = referenceIfAny(signature) ?: return@let
-            if (isFakeOverride) {
+            when {
-                val key = FakeOverrideIdentifier(
+                isFakeOverride -> {
-                    declaration.symbol.unwrapFakeOverrides(),
+                    val key = FakeOverrideIdentifier(
-                    fakeOverrideOwnerLookupTag ?: declaration.containingClassLookupTag()!!
+                        declaration.symbol.unwrapFakeOverrides(),
-                )
+                        fakeOverrideOwnerLookupTag ?: declaration.containingClassLookupTag()!!
-                irFakeOverridesForFirFakeOverrideMap[key] = cachedInSymbolTable
+                    )
-                return cachedInSymbolTable
+                    irFakeOverridesForFirFakeOverrideMap[key] = cachedInSymbolTable
-            } else {
+                    return cachedInSymbolTable
-                error("IR declaration with signature $signature found in SymbolTable and not found in declaration storage")
+                }
+                configuration.useIrFakeOverrideBuilder -> {
+                    /*
+                     * If IR fake override builder is used for building fake-overrides, they are generated bypassing Fir2IrDeclarationStorage,
+                     *   and are written directly to SymbolTable. So in this case it is normal to save the result from symbol table into
+                     *   storage
+                     *
+                     * TODO: potentially this situation won't happen after migration from FIR2IR f/o generator to IR f/o generator (see KT-58861)
+                     */
+                    cache[declaration] = cachedInSymbolTable
+                    return cachedInSymbolTable
+                }
+                else -> {
+                    error("IR declaration with signature $signature found in SymbolTable and not found in declaration storage")
+                }
             }
         }