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

[Thomas Fransham via llvm-commits]

fsfod wrote:

I guess i will have to change my LLVM_C_ABI macro as well although my changes to switch that file to use are not in yet.

 The alternative fix i had before was just a separate macro for namespaces, I was just trying to avoid a proliferation of visibility macros for different code elements.

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