[clang] [Clang] handle both gnu and cpp11 attributes to ensure correct parsing inside extern block (PR #102864)

[Oleksandr T. via cfe-commits]

a-tarasyuk wrote:

@AaronBallman Thanks for the review. I've resolved the conflicts. Could you merge it after the CI passes? Thanks

https://github.com/llvm/llvm-project/pull/102864