FIR: fix position and reporting of PRIVATE_SETTER_*

Previously `FirPropertyAccessor.source` references the `KtProperty` if
the user code contains a setter or getter that doesn't contain a body.
For example, with the following

```
val i: Int = 1
  private set
```

The `FirPropertyAccessor` would reference the `KtProperty` as the
source.

This change makes `FirPropertyAccessor` reference `KtPropertyAccessor`
as the source if possible, regardless of whether the body is present or
not.

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -653,11 +653,11 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = AbstractPropertyWithSetter::class
     }
 
-    abstract class PrivateSetterForAbstractProperty : KtFirDiagnostic<PsiElement>() {
+    abstract class PrivateSetterForAbstractProperty : KtFirDiagnostic<KtModifierListOwner>() {
         override val diagnosticClass get() = PrivateSetterForAbstractProperty::class
     }
 
-    abstract class PrivateSetterForOpenProperty : KtFirDiagnostic<PsiElement>() {
+    abstract class PrivateSetterForOpenProperty : KtFirDiagnostic<KtModifierListOwner>() {
         override val diagnosticClass get() = PrivateSetterForOpenProperty::class
     }
 

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -1056,14 +1056,14 @@ internal class AbstractPropertyWithSetterImpl(
 internal class PrivateSetterForAbstractPropertyImpl(
     firDiagnostic: FirPsiDiagnostic<*>,
     override val token: ValidityToken,
-) : KtFirDiagnostic.PrivateSetterForAbstractProperty(), KtAbstractFirDiagnostic<PsiElement> {
+) : KtFirDiagnostic.PrivateSetterForAbstractProperty(), KtAbstractFirDiagnostic<KtModifierListOwner> {
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
 internal class PrivateSetterForOpenPropertyImpl(
     firDiagnostic: FirPsiDiagnostic<*>,
     override val token: ValidityToken,
-) : KtFirDiagnostic.PrivateSetterForOpenProperty(), KtAbstractFirDiagnostic<PsiElement> {
+) : KtFirDiagnostic.PrivateSetterForOpenProperty(), KtAbstractFirDiagnostic<KtModifierListOwner> {
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }