[llvm] [extract_symbols.py] Add DATA flag for non-function export symbols (PR #101951)

[John Brawn via llvm-commits]

john-brawn-arm wrote:

After a bit of experimentation it looks like the effect that DATA has on the import library is it means the symbol `var` is not defined, only `__imp_var`. This means that if `var` is used without `__declspec(dllimport)` you get a link error instead of incorrect behaviour at runtime.

https://github.com/llvm/llvm-project/pull/101951