Workaround bogus libclang error of supposed unusual deallocations

Add the `-fsized-deallocation` argument to libclang to allow
better compat with GCC 12 / libstdc++ code that uses sized
deallocation in some core areas. Fixes bogus warnings like:

```
call to '__builtin_operator_delete' selects non-usual deallocation function
```

See also:
https://youtrack.jetbrains.com/issue/CPP-29091/In-template-call-to-builtinoperatordelete-selects-non-usual-deallocation-function-gcc-12#focus=Comments-27-6067190.0-0