Switch valptridx error style dispatching from using macro pasting to
using C++11 user-defined literals. This makes the code a bit easier to
read, and removes the need for a C99-conforming preprocessor here, which
should help anyone trying to port to Microsoft Visual Studio.
The new implementation also fixes a limitation of the previous
implementation. Before, an override that referenced an invalid name
could be silently ignored. Now, incorrect overrides cause an attempt to
use an undefined instantiation, which fails with a compilation error.
