Fix double import of _lldb module.

Fix llvm.org/pr39054:
- Register _lldb as a built-in module during initialization of script interpreter,
- Reverse the order of imports in __init__.py: first try to import by absolute name, which will find the built-in module in the context of lldb (and other hosts that embed liblldb), then try relative import, in case the module is being imported from Python interpreter.

This works for SWIG>=3.0.11; before that, SWIG did not support custom module import code.

Differential revision: https://reviews.llvm.org/D52404

llvm-svn: 344474