warning: identifier '_c' preceded by whitespace in a literal operator declaration is deprecated

boost-hana 1.88
clang 20.1.0
c++26

```cxx
// boost/bool.h: 198
namespace literals {
template
constexpr auto operator"" _c() {
return hana::llong({c...})>{};
}
}
```
```bash
/boost/hana/bool.hpp:198:35: warning: identifier '_c' preceded by whitespace in a literal operator declaration is deprecated
```

Start at boost/bool.h line 198 and inspect the literal operator declaration that triggers the Clang 20.1.0 warning under C++26. Confirm the warning with the reported compiler setup, adjust the declaration to remove the deprecated form, and run the existing Boost.Hana test suite to ensure literal behavior is unchanged.

Written by the indexing model from the issue text.