[protobuf/objc] Introduce a protobuf-specific deprecation annotation ⚠️

Instead of using DEPRECATED_ATTRIBUTE from AvailabilityMacros.h, we should introduce a Google-specific Objective-C protobuf deprecation annotation. This helps address IWYU issues with using DEPRECATED_ATTRIBUTE and also enables allows clients to redefine the macro to treat protobuf warnings differently than other types of warnings (e.g., treating protobuf deprecation warnings as errors or ignoring them).

objectivec/GPBBootstrap.h

@@ -91,6 +91,13 @@
 #endif
 #endif
 
+/**
+ * Attribute used for Objective-C proto interface deprecations.
+ **/
+#ifndef GPB_DEPRECATED
+#define GPB_DEPRECATED __attribute__((deprecated))
+#endif
+
 // If property name starts with init we need to annotate it to get past ARC.
 // http://stackoverflow.com/questions/18723226/how-do-i-annotate-an-objective-c-property-with-an-objc-method-family/18723227#18723227
 //

src/google/protobuf/compiler/objectivec/objectivec_helpers.h

@@ -167,7 +167,7 @@ string GetOptionalDeprecatedAttribute(
     isDeprecated = file->options().deprecated();
   }
   if (isDeprecated) {
-    string result = "DEPRECATED_ATTRIBUTE";
+    string result = "GPB_DEPRECATED";
     if (preSpace) {
       result.insert(0, " ");
     }