Gate use of clang nullability qualifiers through absl nullability macros on nullability_on_classes.

This feature is already required for using the clang qualifiers through the absl nullability templates, and avoids breakage in older clang versions.

PiperOrigin-RevId: 738520888
Change-Id: I3ed85ced162ee08e0814c40f33c4d2f8fa677f74

Author: Abseil Team

absl/base/nullability.h

@@ -220,7 +220,8 @@
 // No-op except for being a human readable signal.
 #define ABSL_POINTERS_DEFAULT_NONNULL
 
-#if defined(__clang__) && !defined(__OBJC__)
+#if defined(__clang__) && !defined(__OBJC__) && \
+    ABSL_HAVE_FEATURE(nullability_on_classes)
 // absl_nonnull (default with `ABSL_POINTERS_DEFAULT_NONNULL`)
 //
 // The indicated pointer is never null. It is the responsibility of the provider