Bug 1717448 - Ignore allocations in done within {fmt}. r=sfink

Differential Revision: https://phabricator.services.mozilla.com/D218067

config/check_vanilla_allocations.py

@@ -182,6 +182,10 @@ def main():
         if "_memory_" in filename:
             continue
 
+        # {fmt} can do transient allocations when formatting strings
+        if "third_party_fmt" in filename:
+            continue
+
         # Ignore the fuzzing code imported from m-c
         if "Fuzzer" in filename:
             continue