[llvm] [SandboxIR] Don't add dup instantiation for clang (PR #116387)

[Mészáros Gergely via llvm-commits]

Maetveis wrote:

There is https://github.com/llvm/llvm-project/issues/114308 and https://github.com/llvm/llvm-project/pull/111940#issuecomment-2438776066 discussing this issue. Also I believe the warning is misleading here actually we're hitting https://github.com/llvm/llvm-project/issues/14230.

ping @fsfod btw. I believe we would need to add explicit **declarations** (`extern template`) with the `__declspec(dllexport)` attribute in the source file to upgrade just those two methods to being exported from the explicit template class instantiation. [I mocked this up on compiler explorer here](https://godbolt.org/z/qnc1zorqP), but didn't try it with the LLVM build.

https://github.com/llvm/llvm-project/pull/116387