llvm: Move libprivatelldb to the lldb package

Set PACKAGE=lldb in lldb.pre.mk rather than in individual Makefiles;
change lib/clang/Makefile.inc from PACKAGE=clang to PACKAGE?=clang to
avoid overwriting it.

This is safe to MFC to stable/15 since the moved library will be
picked up automatically by pkg.

MFC after: 2 weeks
Reviewed by: emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56674