Comment out unused error codes and add description for E0316
I have added an extended description of `E0316` and commented out a bunch of unused error codes to make clear the fact that they are no longer in use. You can check for yourself with
```shell
for ec in \
E0314 E0315 E0473 E0474 E0475 E0479 E0480 E0481 \
E0483 E0484 E0485 E0486 E0487 E0488 E0489
do
if [ ! -z "`grep -r $ec compiler/* --exclude-dir=rustc_error_codes`" ]
then
echo $ec
false
fi
done
```
i.e. these error codes appear nowhere in the compiler code and thus cannot be emitted.
r? ```@GuillaumeGomez```
2e8ffed861