[clang] Set dllimport on Objective C ivar offsets (PR #107604)

[Frederik Carlier via cfe-commits]

qmfrederik wrote:

I've added tests to ensure that unmarked ivars are considered protected -- which seems to be the case: https://github.com/llvm/llvm-project/pull/107604/commits/cf93b21734e179144dac34751860b0aa9d2aedaa 

https://github.com/llvm/llvm-project/pull/107604