Migrate template alias nullability annotations to macros.

absl::Nonnull -> absl_nonnull, absl::Nullable -> absl_nullable, and absl::NullabilityUnknown -> absl_nullability_unknown. The new macros are positioned as a qualifier on the affected type, similar to const, rather than as a templated type wrapping the affected type.

See go/cxx-nullability-syntax-change and go/nullability-annotation-alias-to-macro-migration for more details.

Tested:
    Local presubmit tests passed.
PiperOrigin-RevId: 746124354
Change-Id: Ifd8e4b75b42dba251f7d537fe160a03d0332b705