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

[Aaron Ballman via cfe-commits]

================
@@ -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)
----------------
AaronBallman wrote:

`FEATURE` is for standards features, this should be exposed under `EXTENSION` instead.

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