[CMake] More cleanup of installing symlinks.

In order to support building clang out-of-tree the install_symlink script needs to be installed, and it needs to be found by searching the CMAKE_MODULE_PATH.

This change renames install_symlink -> LLVMInstallSymlink so it doesn't conflict with naming from other projects, and adds searching behavior in AddLLVM.cmake

llvm-svn: 248009