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

[via cfe-commits]

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@@ -1016,4 +1016,7 @@ def warn_unpacked_field
 def warn_unaligned_access : Warning<
   "field %1 within %0 is less aligned than %2 and is usually due to %0 being "
   "packed, which can lead to unaligned accesses">, InGroup<UnalignedAccess>, DefaultIgnore;
+
+def err_cannot_mangle_expression : Error<
+  "cannot yet mangle %0 expression">;
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philnik777 wrote:

I think a `// FIXME` would make that a lot more obvious than using the custom diagnostics engine. To me it wasn't obvious that these should be FIXMES, which is the reason I replaced them with a normal diagnostic.

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