[klib] Return the block's return value in inFile methods

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/overrides/FakeOverrideBuilderStrategy.kt

@@ -71,7 +71,7 @@ abstract class FakeOverrideBuilderStrategy(
      * Contract:
      *  * must call [block] exactly once.
      */
-    abstract fun inFile(file: IrFile?, block: () -> Unit)
+    abstract fun <R> inFile(file: IrFile?, block: () -> R): R
 
     /**
      * Callback for creating a symbol for fake override function.

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrUtils.kt

@@ -1305,7 +1305,7 @@ private object BindToNewEmptySymbols : FakeOverrideBuilderStrategy(
         }
     }
 
-    override fun inFile(file: IrFile?, block: () -> Unit) { block() }
+    override fun <R> inFile(file: IrFile?, block: () -> R): R = block()
 }
 
 fun IrClass.addFakeOverrides(