Native: add ExperimentalForeignApi opt-in to performance tests

3f3f6eb marks all cinterop-generated declarations with
@ExperimentalForeignApi. As a result, all usages of such declarations
have to opt-in explicitly. This commit adds those opt-ins to all
usages in the performance tests.

^KT-58362

kotlin-native/performance/numerical/src/main/kotlin-native/launcher.kt

@@ -22,6 +22,7 @@ fun konanBellardPi() {
 
 fun clangBellardPi() {
     for (n in 1 .. 1000 step 9) {
+        @OptIn(kotlinx.cinterop.ExperimentalForeignApi::class)
         val result = cinterop.pi_nth_digit(n)
         Blackhole.consume(result)
     }

kotlin-native/performance/objcinterop/src/main/kotlin-native/org/jetbrains/objCinteropBenchmarks/complexNumbers.kt

@@ -2,6 +2,7 @@
  * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the LICENSE file.
  */
+@file:OptIn(kotlinx.cinterop.ExperimentalForeignApi::class)
 
 package org.jetbrains.complexNumbers
 import kotlinx.cinterop.*

kotlin-native/performance/objcinterop/src/main/kotlin/main.kt

@@ -16,7 +16,10 @@ class ObjCInteropLauncher: Launcher() {
             "fft" to BenchmarkEntryWithInit.create(::ComplexNumbersBenchmark, { fft() })
     )
     override val extendedBenchmarksSet: MutableMap<String, AbstractBenchmarkEntry> = mutableMapOf(
-            "generateNumbersSequence" to BenchmarkEntryWithInit.create(::ComplexNumbersBenchmark, { generateNumbersSequence() }),
+            "generateNumbersSequence" to BenchmarkEntryWithInit.create(::ComplexNumbersBenchmark, {
+                @OptIn(kotlinx.cinterop.ExperimentalForeignApi::class)
+                generateNumbersSequence()
+            }),
             "subComplex" to BenchmarkEntryWithInit.create(::ComplexNumbersBenchmark, { subComplex() }),
             "classInheritance" to BenchmarkEntryWithInit.create(::ComplexNumbersBenchmark, { classInheritance() }),
             "categoryMethods" to BenchmarkEntryWithInit.create(::ComplexNumbersBenchmark, { categoryMethods() }),

kotlin-native/performance/objcinterop/src/main/kotlin/org/jetbrains/objCinteropBenchmarks/complexNumbers.kt

@@ -10,6 +10,7 @@ const val benchmarkSize = 10000
 expect class ComplexNumber
 
 expect class ComplexNumbersBenchmark() {
+    @OptIn(kotlinx.cinterop.ExperimentalForeignApi::class)
     fun generateNumbersSequence(): List<ComplexNumber>
     fun sumComplex()
     fun subComplex()