Install six.py conditionally

Summary:
The current version of LLDB installs six.py into global python library directory. This approach produces conflicts downstream with distribution's py-six copy.

Introduce new configure option LLDB_USE_SYSTEM_SIX (disabled by default). Once specified as TRUE, six.py won't be installed to Python's directory.

Add new option in finishSwigWrapperClasses.py, namely --useSystemSix.

Sponsored by <The NetBSD Foundation>

Reviewers: mgorny, emaste, clayborg, joerg, labath

Reviewed By: labath

Subscribers: #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D29405

llvm-svn: 294071