[llvm] [llvm-shlib] Fix the version naming style of libLLVM for Windows (PR #85710)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

As a headsup to @lazka and @mati865 wrt msys2 packaging: LLVM 18.1.0 and 18.1.1 had the llvm dylib erroneously named `libLLVM.dll`, while this PR restores it to be `libLLVM-18.dll` (and I'm suggesting backporting it).

I didn't include any extra backwards compat symlink named `libLLVM.dll` pointing at `libLLVM-18.dll` though, so any msys2 packages that you've built on top of LLVM 18, that depend on libLLVM, will break and fail to find their dependencies on update of LLVM to later 18.x (if this is backported). So they will need to be rebuilt after restoring the name of the library here.

I guess you can add a temporary symlink/copy within the msys2 packaging of later 18.1.x to smooth out this - but I felt it's the best tradeoff to not provide a symlink on the LLVM side here by default, as symlinks often end up as duplicate copies on Windows.

(This issue with symlinks ending up as large duplicate files can be noticed in the release packages at https://github.com/mstorsjo/llvm-mingw/releases - the `llvm-mingw-*-ucrt-x86_64.zip` packages were around 135 MB each up to 18.1.0 RC2, but after RC3 they grew to 158 MB, as the package contained two separate copies of the libLLVM DLL.)

https://github.com/llvm/llvm-project/pull/85710