Add test_display_source_python() test case to TestSourceManager.py which uses

the lldb PyThon API SBSourceManager to display source files.

To accomodate this, the C++ SBSourceManager API has been changed to take an
lldb::SBStream as the destination for display of source lines.  Modify SBStream::ctor()
so that its opaque pointer is initialized with an StreamString instance.

llvm-svn: 121605