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

[via cfe-commits]

zmodem wrote:

> > It would be nice to keep `GetOrCreateLLVMGlobal` and `GetOrCreateLLVMFunction` in sync. Would it be possible to extract the "Handle dropped DLL attributes" logic into a small utility function that could be called from both?
> 
> I did briefly think about this, but does this change really apply to `GetOrCreateLLVMGlobal`? Is there any test case to exercise it? Perhaps it would be better to leave `GetOrCreateLLVMGlobal` as is?

I think we should look at it the other way around: unless there's a reason that the functions should behave differently (and I'm not aware of any), it seems better for them to keep using the same logic.

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