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

[Thomas Fransham via llvm-commits]

fsfod wrote:

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.

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