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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

Unfortunately, it seems like this isn't enough to fix the build issues:
```
In file included from /Users/martin/code/llvm-project/llvm/lib/Support/BLAKE3/blake3_portable.c:1:
/Users/martin/code/llvm-project/llvm/lib/Support/BLAKE3/blake3_impl.h:186:1: error: expected expression
LLVM_LIBRARY_VISIBILITY
^
/Users/martin/code/llvm-project/llvm/include/llvm/Support/Compiler.h:136:34: note: expanded from macro 'LLVM_LIBRARY_VISIBILITY'
#define LLVM_LIBRARY_VISIBILITY [[gnu::visibility("hidden")]]
                                 ^
In file included from /Users/martin/code/llvm-project/llvm/lib/Support/BLAKE3/blake3_portable.c:1:
/Users/martin/code/llvm-project/llvm/lib/Support/BLAKE3/blake3_impl.h:187:1: error: expected identifier or '('
void blake3_compress_in_place(uint32_t cv[8],
```

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