Create an interface file for SBTarget named SBTarget.i which relieves SBTarget.h

of the duty of having SWIG docstring features and multiline string literals
embedded within.

lldb.swig now %include .../SBTarget.i, instead of .../SBTarget.h.  Will create
other interface files and transition them over.

Also update modify-python-lldb.py to better handle the trailing blank line right
before the ending '"""' Python docstring delimiter.

llvm-svn: 135355