[llvm] llvm-shlib: Fix libLLVM-${MAJOR}.so symlink on MacOS (PR #85163)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Can you try this fix: #85554

Thanks, this fixes the immediate build break.

I can look into fixing the rest of the situation later when I have time (in a day or two). Previously, the effective “soname” on windows was `libLLVM-18.dll`, but now it is `libLLVM.dll`, entirely lacking the version component. (The versioned `libLLVM-18.dll` is installed as a symlink, but the other binaries reference the unversioned name.)

https://github.com/llvm/llvm-project/pull/85163