Insert blocks of python code with swig instead of modify-python-lldb.py

Summary:
Swig is perfectly capable of inserting blocks of python code into its
output (and we use those fascilities already), so there's no need for
this to be done in a post-process step.

lldb_iter is a general-purpose utility used from many classes, so I add
it to the main swig file. The other two blocks are tied to a specific
class, so I add it to the interface file of that class.

Reviewers: zturner, jingham, serge-sans-paille

Subscribers: jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D58350

llvm-svn: 354975