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

[Andrew Ng via cfe-commits]

nga888 wrote:

@zmodem, forgot to ask, and not directly linked to this patch as such, but shouldn't there be a test case added for the dropping of `dllexport`?

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