Cache classloaders for tools.jar and kapt in Gradle workers

#KT-28852 fixed

The loaders and hence some classes were repeatedly loaded and
jit-compiled everytime when KaptExecution were dispatched. Those
classes, like JavaCompiler, can be very large and therefore created a
significant overhead. In some projects, the overhead accounted for more
than 40% of total CPU time of annotation processing.

This change tries to cache the classloaders so that they won't be
reloaded and re-jitted.

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/internal/kapt/KaptWithoutKotlincTask.kt

@@ -100,19 +100,24 @@ private class KaptExecution @Inject constructor(
         private const val JAVAC_CONTEXT_CLASS = "com.sun.tools.javac.util.Context"
 
         private fun kaptClass(classLoader: ClassLoader) = Class.forName("org.jetbrains.kotlin.kapt3.base.Kapt", true, classLoader)
+        private var cachedClassLoaderWithToolsJar: ClassLoader? = null
+        private var cachedKaptClassLoader: ClassLoader? = null
     }
 
     override fun run(): Unit = with(optionsForWorker) {
         val kaptClasspathUrls = kaptClasspath.map { it.toURI().toURL() }.toTypedArray()
         val rootClassLoader = findRootClassLoader()
 
-        val classLoaderWithToolsJar = if (toolsJar != null && !javacIsAlreadyHere()) {
+        val classLoaderWithToolsJar = cachedClassLoaderWithToolsJar ?: if (toolsJar != null && !javacIsAlreadyHere()) {
             URLClassLoader(arrayOf(toolsJar.toURI().toURL()), rootClassLoader)
         } else {
             rootClassLoader
         }
+        cachedClassLoaderWithToolsJar = classLoaderWithToolsJar
+
+        val kaptClassLoader = cachedKaptClassLoader ?: URLClassLoader(kaptClasspathUrls, classLoaderWithToolsJar)
+        cachedKaptClassLoader = kaptClassLoader
 
-        val kaptClassLoader = URLClassLoader(kaptClasspathUrls, classLoaderWithToolsJar)
         val kaptMethod = kaptClass(kaptClassLoader).declaredMethods.single { it.name == "kapt" }
         kaptMethod.invoke(null, createKaptOptions(kaptClassLoader))
     }