[libc++] Reject abilist if it contains an ABI tag (#139030)

We currently don't have any ABI tags in our dylib symbols, and this is
unlikely to change in the future. By diagnosing this we avoid
accidentally adding one through e.g. having `_LIBCPP_HIDE_FROM_ABI` on
an exported symbol.

libcxx/utils/sym_diff.py

@@ -80,6 +80,11 @@ def main():
         old_syms_list, _ = util.filter_stdlib_symbols(old_syms_list)
         new_syms_list, _ = util.filter_stdlib_symbols(new_syms_list)
 
+    for symbol in new_syms_list:
+        if symbol["is_defined"] and 'B' in symbol["name"]:
+            print(f"Symbol {symbol['name']} contains an ABI tag!")
+            sys.exit(1)
+
     added, removed, changed = diff.diff(old_syms_list, new_syms_list)
     if args.removed_only:
         added = {}