rustc: Unify fat pointer ABI constants.

This merges the `trt_field_*`, `fn_field_*` and `slice_elt_*` constants
into two `FAT_PTR_*` constants. This resolves the first part of #18590.
rustc: Change all ABI constants to all caps.

This also removes `box_field_refcnt`. It was not used anywhere.