Avoid repeated format of union members (#22297)

Codex claims this makes type checking pandas-stubs like 10% faster

Co-authored-by Codex

Author: hauntsaninja

mypy/messages.py

@@ -2678,15 +2678,22 @@ def format_list(types: Sequence[Type]) -> str:
         return ", ".join(format(typ) for typ in types)
 
     def format_union_items(types: Sequence[Type]) -> list[str]:
-        formatted = [format(typ) for typ in types if format(typ) != "None"]
+        formatted = []
+        has_none = False
+        for typ in types:
+            item = format(typ)
+            if item == "None":
+                has_none = True
+            else:
+                formatted.append(item)
         if len(formatted) > MAX_UNION_ITEMS and verbosity == 0:
             more = len(formatted) - MAX_UNION_ITEMS // 2
             formatted = formatted[: MAX_UNION_ITEMS // 2]
         else:
             more = 0
         if more:
             formatted.append(f"<{more} more items>")
-        if any(format(typ) == "None" for typ in types):
+        if has_none:
             formatted.append("None")
         return formatted