Identified with modernize-use-nullptr.

We dropped downstream support for Python 2 in the previous release. Now
that we have branched for the next release the window where this kind of
change could introduce conflicts is closing too. Start by getting rid of
Python 2 support in the Script Interpreter plugin.

Differential revision: https://reviews.llvm.org/D124429

The C headers are deprecated so as requested in D102845, this is replacing them
all with their (not deprecated) C++ equivalent.

Reviewed By: shafik

Differential Revision: https://reviews.llvm.org/D103084

The size is known and the truncation is deliberate -- use memcpy instead
of strncpy.

This is a follow-up to https://reviews.llvm.org/D69793

Fix https://bugs.llvm.org/show_bug.cgi?id=43830 while avoiding polluting the
global Python namespace.

This both reverts r357277 to rebundle a version of Python's readline module
based on libedit.

However, this patch also provides two improvements over the previous
implementation:

1. use PyMem_RawMalloc instead of PyMem_Malloc, as expected by PyOS_Readline
   (prevents to segfault upon exit of interactive session)
2. patch the readline module upon embedded interpreter loading, instead of
   patching it globally, which should prevent any side effect on other
   modules/packages
3. only activate the patched module if libedit is actually linked in lldb

Differential Revision: https://reviews.llvm.org/D69793