[llvm] Initial changes for llvm shared library build using explicit visibility annotations (PR #96630)

[Thomas Fransham via llvm-commits]

fsfod wrote:

Would changing the preprocessor condition from 
``` c++
#if LLVM_HAS_CPP_ATTRIBUTE(gnu::visibility) && defined(__GNUC__) &&            \
    !defined(__clang__)
```
to 
``` c++
#if LLVM_HAS_CPP_ATTRIBUTE(gnu::visibility) && defined(__GNUC__) &&            \
    (!defined(__clang__) || _clang_major__ < 15)
``` 

not be enough to fix it?

https://github.com/llvm/llvm-project/pull/96630