[clang] [Clang] Add __datasizeof (PR #67805)

[Aaron Ballman via cfe-commits]

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@@ -236,6 +236,7 @@ FEATURE(shadow_call_stack,
 FEATURE(tls, PP.getTargetInfo().isTLSSupported())
 FEATURE(underlying_type, LangOpts.CPlusPlus)
 FEATURE(experimental_library, LangOpts.ExperimentalLibrary)
+FEATURE(datasizeof, LangOpts.CPlusPlus)
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AaronBallman wrote:

Yeahhhhh, but we still need to diagnose it as an extension because that's what it is. (We should not add extensions that we do not diagnose under `-pedantic` -- we have been inconsistent in the past but we've gotten better in the past several years.)

Pedantic warnings aren't triggered in system headers, so I think I'm failing to see the concern: https://godbolt.org/z/nn7TdKrcv

https://github.com/llvm/llvm-project/pull/67805