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

This reverts the changes from 91a38462
for Windows targets. The changes in that commit don't work as expected
for Windows targets (those parts of llvm_add_library don't quite behave
the same for Windows), while the previous status quo (producing a
library named "libLLVM-<major>.dll") is the defacto standard way of
doing versioned library names there, contrary to on Unix.

After that commit, the library always ended up named "libLLVM.dll",
executables linking against it would reference "libLLVM.dll", and
"libLLVM-<major>.dll" was provided as a symlink.

Thus revert this bit back to as it were, so that executables actually
link against a versioned libLLVM, and no separate symlink is needed.

The only thing that might be improved compared to the status quo as it
was before these changes, is that the import library is named
"lib/libLLVM-<major>.dll.a", while the common style would be to name it
plainly "lib/libLLVM.dll.a" (even while it produces references to
"libLLVM-<major>.dll", but none of these had that effect for Windows
targets.

(As a side note, the llvm-shlib library can be built for MinGW, but not
currently in MSVC configurations.)

(cherry picked from commit cb2ca233)