Very minor refactoring to ease development and debugging.

Having to declare a local `std::string` has bugged me many times. Nice to get this little nuisance out of the way.

Extracted from PR #4022, where it is used like this:

```
    std::fprintf(stdout,
                 "\nTYPE_CASTER_ODR_GUARD_IMPL %s %s\n",
                 clean_type_id(intrinsic_type_info.name()).c_str(),
                 source_file_line_from_sloc.c_str());
```