[clang] [clang][Sema] Fix typo in 'offsetof' diagnostics (PR #133448)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Good catch! Changes LGTM with a minor change to the diagnostic to quote the construct.

If we wanted to improve the diagnostic wording further, it might help to say "is a Clang extension" so the user understands why the diagnostic is being emitted (otherwise the wording just explains what the code is doing). But I don't insist on a change like that.

https://github.com/llvm/llvm-project/pull/133448