types when they are available.

This makes them interchangeable in contexts known to be compiled as C++20.
This also makes `absl::` ordering types compatible with `<=>`, allowing to
unconditionally use `absl::` spelling for types but conditionally use `<=>`
when available.

PiperOrigin-RevId: 588085408
Change-Id: I1aa5247f0e31acbb838ee76829b7a13c74b0a94f