[clang] [clang] Fix loss of `dllexport` for exported template specialization (PR #94664)

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zmodem wrote:

> One function handles "functions" and the other "globals", so behaviour is not necessarily the same. However, if you feel it would be better that both use the same logic, then I will go ahead with adding this.

Yes, I think that since the dll attribute logic has been the same between GetOrCreateLLVMGlobal and GetOrCreateLLVMFunction so far, we should keep it the same until we find an explicit reason to make them different.

https://github.com/llvm/llvm-project/pull/94664