[clang] [Clang][Sema] Add -Wredundant-defer to diagnose redundant `_Defer`s (PR #225748)

[Ambrose Leeb via cfe-commits]

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@@ -11342,6 +11342,10 @@ def warn_empty_defer_body : Warning<
 def note_empty_body_on_separate_line : Note<
   "put the semicolon on a separate line to silence this warning">;
 
+def warn_redundant_defer: Warning<
+  "redundant use of defer; the deferred statement is executed immediately">,
+  InGroup<RedundantDefer>, DefaultIgnore;
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Sirraide wrote:

The diagnostic should either be on by default or part of something like `-Wall`; adding off by default warnings is something we try to avoid nowadays because usually no-one turns them on.

https://github.com/llvm/llvm-project/pull/225748