[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@248043 91177308-0d34-0410-b5e6-96231b3b80d8