[CMake] Fixing a small hack in add_lldb_library

This code was adding an explicit dependency on libclang because lldb needs clang headers, changing this to instead depend on the clang tablegen targets means we don't have to depend on building the clang bits in libclang that lldb doesn't need.

Note this is still a bit of a hack because we're adding the dependency to all lldb libraries, instead of just the ones that need it.

llvm-svn: 282196