[CMake] More cleanup of installing symlinks.

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248009 91177308-0d34-0410-b5e6-96231b3b80d8