[FIR] Introduce the PluginGenerated source kind

Make parcelize work consistently with
`FirOverrideChecker`.

Actually, there is a philosophical question about
what to do when a plugin generates a class
in a new package, and this class is red code
(for instance, doesn't implement an abstract
member from a supertype). There's no source
to report such an error, but we probably do
want to run checkers to avoid trying to
compile red code to binaries (because it may
crash in backend, or it may silently work).

compiler/frontend.common/src/org/jetbrains/kotlin/KtSourceElement.kt

@@ -309,6 +309,9 @@ sealed class KtFakeSourceElementKind(final override val shouldSkipErrorTypeRepor
 
     // For it.functionFromAny() calls on a stub type
     object CastToAnyForStubTypes : KtFakeSourceElementKind()
+
+    // For plugin-generated things
+    object PluginGenerated : KtFakeSourceElementKind()
 }
 
 sealed class AbstractKtSourceElement {