Build libclang also if LLDB is enabled

LLDB depends on libclang as it uses Clang as the expression parser.
Previously setting WITHOUT_CLANG but leaving LLDB enabled (as default)
resulted in a build failure.

Users who set WITHOUT_CLANG in order to reduce build time or size
might want to set WITHOUT_LLDB in addition to WITHOUT_CLANG, or use
WITHOUT_TOOLCHAIN instead.

PR: 260993
Reported by: eugen
Reviewed by: dim
MFC after: 3 days
Sponsored by: The FreeBSD Foundation