Convert the ScriptInterpreter system to a plugin-based one.

Convert the ScriptInterpreter system to a plugin-based one.

Previously embedded interpreters were handled as ad-hoc source
files compiled into source/Interpreter.  This made it hard to
disable a specific interpreter, or to add support for other
interpreters and allow the developer to choose which interpreter(s)
were enabled for a particular build.

This patch converts script interpreters over to a plugin-based system.
Script interpreters now live in source/Plugins/ScriptInterpreter, and
the canonical LLDB interpreter, ScriptInterpreterPython, is moved there
as well.

Any new code interfacing with the Python C API must live in this location
from here on out.  Additionally, generic code should never need to
reference or make assumptions about the presence of a specific interpreter
going forward.

Differential Revision: http://reviews.llvm.org/D11431
Reviewed By: Greg Clayton

llvm-svn: 243681