Delete output when unable to run KAPT incrementally

When comparing previous and current classpath snapshots, and it is not
possible to compute the types that changed, make sure to delete the outputs
before invoking KAPT.

Test: KaptIncrementalWithAggregatingApt.testIncompatibleClasspathChanges

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/KaptIncrementalWithAggregatingApt.kt

@@ -9,6 +9,7 @@ import org.jetbrains.kotlin.gradle.util.modify
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertTrue
 import org.junit.Test
+import kotlin.test.assertFalse
 
 class KaptIncrementalWithAggregatingApt : KaptIncrementalIT() {
 
@@ -136,4 +137,37 @@ class KaptIncrementalWithAggregatingApt : KaptIncrementalIT() {
             assertTrue(getProcessedSources(output).isEmpty())
         }
     }
+
+    @Test
+    fun testIncompatibleClasspathChanges() {
+        val project = getProject()
+        project.projectFile("useB.kt").modify { current ->
+            current + """
+                
+                @example.ExampleAnnotation
+                fun addedFunctionB() = ""
+            """.trimIndent()
+        }
+        project.build("clean", "build") {
+            assertSuccessful()
+        }
+
+        project.projectFile("useB.kt").modify { current ->
+            current.replace("fun addedFunctionB", "fun renamedFunctionB")
+        }
+        project.gradleBuildScript().appendText("""
+            
+            dependencies {
+                compile 'com.google.guava:guava:12.0'
+            }
+        """.trimIndent())
+        project.build("build") {
+            assertSuccessful()
+
+            assertFalse(
+                fileInWorkingDir("build/generated/source/kapt/main/bar/AddedFunctionBGenerated.java").exists(),
+                "Generated file should be deleted for renamed function when classpath changes."
+            )
+        }
+    }
 }