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
