[ObjC] Mark the syntax on FileDescriptor as deprecated.

This really shouldn't be used for making decisions, so mark it as such.

PiperOrigin-RevId: 488683993

objectivec/GPBDescriptor.h

@@ -133,8 +133,16 @@ typedef NS_ENUM(uint8_t, GPBFieldType) {
 @property(nonatomic, readonly, copy) NSString *package;
 /** The objc prefix declared in the proto file. */
 @property(nonatomic, readonly, copy, nullable) NSString *objcPrefix;
-/** The syntax of the proto file. */
-@property(nonatomic, readonly) GPBFileSyntax syntax;
+/**
+ * The syntax of the proto file.
+ *
+ * This should not be used for making decisions about support
+ * features/behaviors, what proto2 vs. proto3 syntax has meant has evolved over
+ * time, and not more specific methods on the descriptors should be used
+ * instead.
+ */
+@property(nonatomic, readonly) GPBFileSyntax syntax
+    __attribute__((deprecated("Syntax is not a good way to decide things about behaviors.")));
 
 @end
 

objectivec/GPBDescriptor.m

@@ -505,8 +505,11 @@ uint32_t GPBFieldAlternateTag(GPBFieldDescriptor *self) {
       //  - not repeated/map
       //  - not in a oneof (negative has index)
       //  - not a message (the flag doesn't make sense for messages)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
       BOOL clearOnZero = ((file.syntax == GPBFileSyntaxProto3) && !isMapOrArray &&
                           (coreDesc->hasIndex >= 0) && !isMessage);
+#pragma clang diagnostic pop
       if (clearOnZero) {
         coreDesc->flags |= GPBFieldClearHasIvarOnZero;
       }
@@ -521,7 +524,10 @@ uint32_t GPBFieldAlternateTag(GPBFieldDescriptor *self) {
       // to be what the runtime was doing (even though it was wrong). This is
       // only wrong in the rare cases an enum is declared in a proto3 syntax
       // file but used for a field in the proto2 syntax file.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
       BOOL isClosedEnum = (dataType == GPBDataTypeEnum && file.syntax != GPBFileSyntaxProto3);
+#pragma clang diagnostic pop
       if (isClosedEnum) {
         coreDesc->flags |= GPBFieldClosedEnum;
       }