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

[Aaron Ballman via cfe-commits]

================
@@ -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">;
----------------
AaronBallman wrote:

All of the other "cannot yet mangle" diagnostics are done with a custom diagnostic, as in https://github.com/llvm/llvm-project/blob/475e154331af19f175ec082b08547b155bba1577/clang/lib/AST/ItaniumMangle.cpp#L5116 -- I think we can drop this diagnostic and do the same (that helps make it more obvious in the code that the diagnostic is really a fixme issue rather than a diagnostic we intend to keep long-term). WDYT?

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