[clang] [clang] Fix loss of `dllexport` for exported template specialization (PR #93302)

[Andrew Ng via cfe-commits]

nga888 wrote:

> Does the test exercise both modified paths? I'm guessing it only exercises GetOrCreateLLVMFunction, but I'm not a codegen expert.

Only 1 path is "exercised" but I believe neither path of the removed code was exercised previously and no other tests are affected by the code removal.

https://github.com/llvm/llvm-project/pull/93302