FIR: fix incorrect psi for named & spread arguments in FIR builder

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2000-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -237,13 +237,13 @@ class RawFirBuilder(
             val isSpread = getSpreadElement() != null
             return when {
                 name != null -> buildNamedArgumentExpression {
-                    source = expression?.toFirSourceElement()
+                    source = (this@toFirExpression as? PsiElement)?.toFirSourceElement()
                     this.expression = firExpression
                     this.isSpread = isSpread
                     this.name = name
                 }
                 isSpread -> buildSpreadArgumentExpression {
-                    source = expression?.toFirSourceElement()
+                    source = (this@toFirExpression as? PsiElement)?.toFirSourceElement()
                     this.expression = firExpression
                 }
                 else -> firExpression