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

[Frederik Carlier via cfe-commits]

qmfrederik wrote:

@davidchisnall Would you mind reviewing this PR?  This came up when trying to build gnustep-gui using the Windows-native version of LLVM/clang (i.e., not in MSYS).

The problem is that all the ivar references are missing the `dllimport` annotation.  This is not a problem on MSYS2 because the linker detects this and will use pseudo relocations to correct for this.  This does not happen on a 'native' Windows toolchain, though.

I can confirm that gnustep-gui builds after implementing this patch.

https://github.com/gnustep/tools-windows-msvc/issues/36#issuecomment-2333950722 has more information.

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