Move to debug assert

src/libraries/Common/src/Roslyn/CSharpSyntaxHelper.cs

@@ -79,7 +79,7 @@ public override void AddAliases(SyntaxNode node, ref ValueListBuilder<(string al
             }
             else
             {
-                throw new System.InvalidOperationException("Unreachable");
+                Debug.Assert(false, "This should not be reachable.  Caller already checked we had a compilation unit or namespace.");
             }
         }