Fix ArgumentException in SeparatedSyntaxList.ReplaceSeparator to use proper message and paramName

src/Compilers/Core/Portable/CodeAnalysisResources.resx

@@ -438,6 +438,12 @@
   <data name="MissingListItem" xml:space="preserve">
     <value>The item specified is not the element of a list.</value>
   </data>
+  <data name="SeparatorTokenMustHaveSameRawKind" xml:space="preserve">
+    <value>New separator must have the same RawKind as the separator being replaced.</value>
+  </data>
+  <data name="SeparatorTokenMustHaveSameLanguage" xml:space="preserve">
+    <value>New separator must have the same Language as the separator being replaced.</value>
+  </data>
   <data name="InvalidPublicKey" xml:space="preserve">
     <value>Invalid public key.</value>
   </data>

src/Compilers/Core/Portable/Syntax/SeparatedSyntaxList.cs

@@ -573,13 +573,17 @@ public SeparatedSyntaxList<TNode> ReplaceSeparator(SyntaxToken separatorToken, S
             var index = nodesWithSeps.IndexOf(separatorToken);
             if (index < 0)
             {
-                throw new ArgumentException("separatorToken");
+                throw new ArgumentException(CodeAnalysisResources.MissingListItem, nameof(separatorToken));
             }
 
-            if (newSeparator.RawKind != nodesWithSeps[index].RawKind ||
-                newSeparator.Language != nodesWithSeps[index].Language)
+            if (newSeparator.RawKind != nodesWithSeps[index].RawKind)
             {
-                throw new ArgumentException("newSeparator");
+                throw new ArgumentException(CodeAnalysisResources.SeparatorTokenMustHaveSameRawKind, nameof(newSeparator));
+            }
+
+            if (newSeparator.Language != nodesWithSeps[index].Language)
+            {
+                throw new ArgumentException(CodeAnalysisResources.SeparatorTokenMustHaveSameLanguage, nameof(newSeparator));
             }
 
             return new SeparatedSyntaxList<TNode>(nodesWithSeps.Replace(separatorToken, newSeparator));