This is a followup to the [previous
change](https://github.com/abseil/abseil-cpp/commit/69195d5bd2416a7224416887c78353ee8edf67ee)
that added the `ABSL_NULLABILITY_COMPATIBLE` attribute macro.

Adding these attributes has the following benefits:

- Clang itself can now diagnose certain nullability errors through the
  `-Wnonnull` and `-Wnullability` warnings.

- The nullability annotations can now also be used on pointers to incomplete
  types, as we have removed the `IsSupportedType` mechanism that used the
  `absl_nullability_compatible` tag to check whether a type is
  nullability-compatible (which only worked for complete types) and instead let
  Clang perform this check through the `ABSL_NULLABILITY_COMPATIBLE` attribute
  (which also works on incomplete types).

PiperOrigin-RevId: 684342145
Change-Id: I94c8affd5be704cb49340058ced177f09ebd83a3