[SandboxIR] Don't add dup instantiation for clang

[jsji]

On Windows some clang-based compiler(eg: clang-cl, icx) also define _MSC_VER.

[Maetveis]

There is #114308 and #111940 (comment) discussing this same issue ping @fsfod, @bogner. I believe the warning is misleading here, I think we're hitting #14230 (comment).

Taking a deeper look, I believe we would need to add explicit declarations (extern template) with the __declspec(dllexport) attribute in the source file in addition to the (import) declaration in the headers to upgrade just those two methods to being exported from the explicit template class instantiation. I mocked this up on compiler explorer here, but didn't try it with the LLVM build.

[fsfod]

I'm not sure for which you mean that need to have extern templates for that don't already have them.
For now a quick fix for now could be to change the define to #if defined(_MSC_VER) && defined(LLVM_BUILD_LLVM_DYLIB) so no one doing normal builds should get the warnings. I also realized the explicit function instantiations in source file need to be moved above the class explicit template instantiation definition for clang-cl to actually export them.