[llvm] [FileCheck] Add missing export to symbol used in test (PR #225978)

[Tomohiro Kashiwada via llvm-commits]

kikairoya wrote:

MSVC doesn't allow adding `dllexport`.
Instead, `LLVM_ABI` is required on the declaration in the header.
https://godbolt.org/z/c1zrq8KcY

https://github.com/llvm/llvm-project/pull/225978