[CMake] clang-*.*: Prevent versioning if the buildhost is targeting for Win32.

CMake-2.8.12 is hardcoded to create symlinked clang.exe if the target property VERSION is present and the host is not Win32.
Then clang.exe-*.* is generated and clang.exe is symlinked to it.

  lrwxrwxrwx. 1 bb bb      13 Jul  5 18:04 clang.exe -> clang.exe-3.7
  -rwxr-x---. 1 bb bb  244763 Jul  5 18:04 clang++.exe
  -rwxr-x---. 1 bb bb  244763 Jul  5 18:04 clang.exe-3.7

It made me unhappy when built binaries were copied to the Windows target.

FIXME: Could we just remove the target property VERSION in add_llvm_executable() ?
llvm-svn: 241403