Ensure direcory for class file stubs always exists

This fixes the warning "classpath entry does not exist"

    #KT-12352 fixed

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

@@ -46,6 +46,7 @@ fun Project.initKapt(
         kotlinTask.logger.kotlinDebug("kapt: Using class file stubs")
 
         val stubsDir = File(buildDir, "tmp/kapt/$variantName/classFileStubs")
+        stubsDir.mkdirs()
         kotlinTask.extensions.extraProperties.set("kaptStubsDir", stubsDir)
         javaTask.appendClasspathDynamically(stubsDir)
         kotlinTask.appendClasspathDynamically(stubsDir)

libraries/tools/kotlin-gradle-plugin/src/test/kotlin/org/jetbrains/kotlin/gradle/KaptIT.kt

@@ -68,6 +68,7 @@ class KaptIT: BaseGradleIT() {
             assertFileExists("build/classes/main/example/SourceAnnotatedTestClassGenerated.class")
             assertFileExists("build/classes/main/example/BinaryAnnotatedTestClassGenerated.class")
             assertFileExists("build/classes/main/example/RuntimeAnnotatedTestClassGenerated.class")
+            assertNotContains("w: Classpath entry points to a non-existent location")
         }
 
         project.build("build") {