No diagnostic for redefinition of symbols

## cdecl+ Input
```cpp
size_t size_t
```

## Expected Prose
> Error: redefinition of `size_t` as different kind of symbol

## Actual Prose or Error
> Declare `size_t` as `size_t`

## Additional Context
Any use of a `_t` type as a *typedef-name* should register that symbol as a known type, and using the same symbol as an identifier in any context should then be illegal.