CMake Xcode builds: symlink tblgen targets out to bin/.

Xcode always puts executable targets in the directory
bin/<Config>. When building separate LLVM and Clang projects for
Xcode, this prevents the CMake-configured project for Clang from
finding llvm-tblgen. Add a symlink so that tblgen executables are
always available in bin/ (regardless of the configuration LLVM is
built with).