FIR: allow diagnostic collection on default property accessors

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirSourceElement.kt

@@ -314,6 +314,7 @@ class FirFakeSourceElement(psi: PsiElement, override val kind: FirFakeSourceElem
 }
 
 fun FirSourceElement.fakeElement(newKind: FirFakeSourceElementKind): FirSourceElement {
+    if (kind == newKind) return this
     return when (this) {
         is FirLightSourceElement -> FirLightSourceElement(lighterASTNode, startOffset, endOffset, treeStructure, newKind)
         is FirPsiSourceElement -> FirFakeSourceElement(psi, newKind)

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirDefaultPropertyAccessor.kt

@@ -8,9 +8,7 @@ package org.jetbrains.kotlin.fir.declarations.impl
 import org.jetbrains.kotlin.descriptors.EffectiveVisibility
 import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.descriptors.Visibility
-import org.jetbrains.kotlin.fir.FirImplementationDetail
-import org.jetbrains.kotlin.fir.FirModuleData
-import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.*
 import org.jetbrains.kotlin.fir.contracts.impl.FirEmptyContractDescription
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.builder.buildDefaultSetterValueParameter
@@ -126,7 +124,7 @@ class FirDefaultPropertySetter(
     FirImplicitUnitTypeRef(source),
     valueParameters = mutableListOf(
         buildDefaultSetterValueParameter builder@{
-            this@builder.source = source
+            this@builder.source = source?.fakeElement(FirFakeSourceElementKind.DefaultAccessor)
             this@builder.moduleData = moduleData
             this@builder.origin = origin
             this@builder.returnTypeRef = propertyTypeRef