lldb: add rule to generate LLDBWrapLua.cpp

Building lldb's lua/python bindings requires swig, but we do not want to
include it in the FreeBSD base system (as a build tool) because it has
non-trivial dependencies. As a workaround, add a make rule to generate
LLDBWrapLua.cpp, and we will commit the generated file.

Requires the swig30 package.

Reviewed by: brooks
Discussed with: dim
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24265