[FIR] Use ConeKotlinType to represent vararg's element type

We are using `ConeKotlinType` instead of `FirTypeRef` to represent
that element type of vararg doesn't have any source. It has a type
that was inferred. If we try to specify a source, then we could
end up with the incorrect place for diagnostic.

#KT-59682 Fixed

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -616,7 +616,7 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
 
         varargArgumentsExpression.configure {
             +fieldList("arguments", expression)
-            +field("varargElementType", typeRef)
+            +field("coneElementTypeOrNull", coneKotlinTypeType, nullable = true)
         }
 
         samConversionExpression.configure {