[clang] [clang] Implement -Walloc-size diagnostic option (PR #150028)

[Erich Keane via cfe-commits]

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@@ -3684,6 +3684,12 @@ def warn_alloca_align_alignof : Warning<
   "second argument to __builtin_alloca_with_align is supposed to be in bits">,
   InGroup<DiagGroup<"alloca-with-align-alignof">>;
 
+def warn_alloc_size
+    : Warning<
+          "allocation of insufficient size '%0' for type %1 with size '%2'">,
+      InGroup<DiagGroup<"alloc-size">>,
+      DefaultIgnore;
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erichkeane wrote:

We tend to not really want warnings to be default-ignore.  @AaronBallman : Should we at least add this to `Wextra`?

https://github.com/llvm/llvm-project/pull/150028