[CMake] Refactoring and cleaning up clang symlink generation.

Generation of clang symlinks now uses add_clang_symlink macro which uses add_llvm_symlink. Also the list of symlinks to generate is configurable via CLANG_LINKS_TO_CREATE.

This re-lands r248015, with fixes for clang symlinks to always be generated as part of the clang and install-clang targets.

llvm-svn: 248043