Kapt3: Write stub class files with metadata (needed for incremental compilation)

compiler/testData/codegen/light-analysis/boxingOptimization/safeCallWithElvis.txt

@@ -1,3 +1,4 @@
+@kotlin.Metadata
 public final class A {
     private final field x: int
     private final @org.jetbrains.annotations.Nullable field y: A
@@ -6,6 +7,7 @@ public final class A {
     public final @org.jetbrains.annotations.Nullable method getY(): A
 }
 
+@kotlin.Metadata
 public final class SafeCallWithElvisKt {
     public final static @org.jetbrains.annotations.NotNull method box(): java.lang.String
     public final static method check(@org.jetbrains.annotations.Nullable p0: A): int