[llvm] [SandboxIR] Add extern templates for GlobalWithNodeAPI (PR #111940)

[Thomas Fransham via llvm-commits]

fsfod wrote:

I have double checked again for shared library debug builds with clang-cl and the template function symbols are not exported without them being explicitly exported from the source file, explicit template instantiation of containing classes is not enough. I could add some clang warning suppression macros around them though. 
My branch with all my stuff still to merge just for clang-cl is [here ](https://github.com/fsfod/llvm-project/commits/exported-api/llvm-20-clang-cl/) that I checked with.

https://github.com/llvm/llvm-project/pull/111940