We use CARBON_KIND_SWITCH for handling the output of TypeIterator in the
TypeStructureBuilder
Here is how the errors look when it is misused:
- If you don't cover ever type in the variant with a case
```
Enumeration value 'VariantTypeT1NotHandledInSwitch' not handled in
switch
```
Is attached to the CARBON_KIND_SWITCH() usage, the `T1` being a 0-based
index into the std::variant's type list, indicating which type was
missed.
- If you have a case for a type that is not in the variant
```
In template: constraints not satisfied for class template
'ValidCaseType' [with T = char]
... bunch of instantiation stuff ...
kind_switch.h(124, 12): Because 'char' does not satisfy
'TypeFoundInVariant'
```
Where `char` was the type I put in the `CARBON_KIND` macro, which was
not in the variant.
- If you have too many types in your variant (currently > 12)
```
In template: static assertion failed due to requirement 'sizeof...(Ts)
<= 12': CARBON_KIND_SWITCH supports std::variant with up to 12 types.
Add more if needed.
```
Is attached to the CARBON_KIND_SWITCH() usage.
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
