VS: Encode newlines in XML attributes

Encode `\n` as `
` to avoid generating a literal newline inside an
XML attribute.  This is more readable and also fixes custom commands in
`.csproj` files with VS 2019 RC.

Fixes: #19001

Source/cmVisualStudio10TargetGenerator.cxx

@@ -35,6 +35,7 @@ static std::string cmVS10EscapeAttr(std::string arg)
   cmSystemTools::ReplaceString(arg, "<", "&lt;");
   cmSystemTools::ReplaceString(arg, ">", "&gt;");
   cmSystemTools::ReplaceString(arg, "\"", "&quot;");
+  cmSystemTools::ReplaceString(arg, "\n", "&#10;");
   return arg;
 }