lldb: add rule to generate LLDBWrapLua.cpp

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