FIR: make class delegation field a fake source

Previously, the FirFieldImpl generated from class delegation has a real
source. This causes troubles in the IDE. This change adds a new fake
source kind for class delegation fields.

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt

@@ -1574,7 +1574,7 @@ class DeclarationsConverter(
         val delegateName = Name.special("<\$\$delegate_${delegateFields.size}>")
         delegateFields.add(
             buildField {
-                source = firExpression!!.source
+                source = firExpression!!.source?.fakeElement(FirFakeSourceElementKind.ClassDelegationField)
                 declarationSiteSession = baseSession
                 origin = FirDeclarationOrigin.Synthetic
                 name = delegateName

compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt

@@ -566,7 +566,8 @@ open class RawFirBuilder(
                         val delegateExpression = { superTypeListEntry.delegateExpression }.toFirExpression("Should have delegate")
                         container.superTypeRefs += type
                         val delegateName = Name.special("<\$\$delegate_${delegateFields.size}>")
-                        val delegateSource = superTypeListEntry.delegateExpression?.toFirSourceElement()
+                        val delegateSource =
+                            superTypeListEntry.delegateExpression?.toFirSourceElement(FirFakeSourceElementKind.ClassDelegationField)
                         val delegateField = buildField {
                             source = delegateSource
                             declarationSiteSession = baseSession

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

@@ -170,6 +170,10 @@ sealed class FirFakeSourceElementKind : FirSourceElementKind() {
     // for java annotations constructor implicit parameters are generated
     // with a fake source which refers to declared annotation methods
     object ImplicitAnnotationAnnotationConstructorParameter : FirFakeSourceElementKind()
+
+    // for the implicit field storing the delegated object for class delegation
+    // with a fake source that refers to the KtExpression that creates the delegate
+    object ClassDelegationField : FirFakeSourceElementKind()
 }
 
 sealed class FirSourceElement {