[llvm] [clang] [Offload] Fix the offloading wrapper when merged multiple times. (PR #79231)

[Johannes Doerfert via cfe-commits]

https://github.com/jdoerfert commented:

Do we need two different linkages or could the COFF setting be used in both?
Can we have a test to show the merging works as expected?

https://github.com/llvm/llvm-project/pull/79231