[llvm] Fix compiler errors in GVN.h for clang versions older than 15 (PR #107636)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

And just to be clear - the issue here is that `LLVM_LIBRARY_VISIBILITY` now expands to a C++ attribute, but we're using it in a couple of pure C files, which don't support C++ style attributes.

https://github.com/llvm/llvm-project/pull/107636