Embed swig version into lldb.py in a different way

Summary:
Instead of doing string chopping on the resulting python file, get swig
to output the version for us. The two things which make slightly
non-trivial are:
- in order to get swig to expand SWIG_VERSION for us, we cannot use
  %pythoncode directly, but we have to go through an intermediate macro.
- SWIG_VERSION is a hex number, but it's components are supposed to be
  interpreted decimally, so there is a bit of integer magic needed to
  get the right number to come out.

I've tested that this approach works both with the latest (3.0.12) and
oldest (1.3.40) supported swig.

Reviewers: zturner, jingham, serge-sans-paille

Subscribers: jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D58172

llvm-svn: 354104