modify-python-lldb.py: clean up __iter__ and __len__ support

Summary:
Instead of modifying the swig-generated code, just add the appropriate
methods to the interface files in order to get the swig to do the
generation for us.

This is a straight-forward move from the python script to the interface
files. The single class which has nontrivial handling in the script
(SBModule) has been left for a separate patch.

For the cases where I did not find any tests exercising the
iteration/length methods (i.e., no tests failed after I stopped emitting
them), I tried to add basic tests for that functionality.

Reviewers: zturner, jingham, amccarth

Subscribers: jdoerfert, lldb-commits

Differential Revision: https://reviews.llvm.org/D60119

llvm-svn: 357572