[CMake] NFC. Add clang-tablegen-targets utility target

By creating this target other projects that depend on clang-generated headers (like LLDB) will no longer be order-dependent on Clang being processed by CMake first.

Also, by creating a dummy of this target in ClangConfig.cmake, projects that can build against out-of-tree clang can freely depend on the target without needing to have conditionals for if clang is in-tree or out-of-tree.

llvm-svn: 309390