[CMake] Support symlinks even with LLVM_INSTALL_TOOLCHAIN_ONLY

When LLVM_INSTALL_TOOLCHAIN_ONLY is used and LLVM_TOOLCHAIN_TOOLS
contains a tool which is a symlink, it would be ignored. This already
worked before but got broken in r282510.

Differential Revision: https://reviews.llvm.org/D25067

llvm-svn: 282844