Big Kaleidoscope tutorial update.

This commit switches the underlying JIT for the Kaleidoscope tutorials from
MCJIT to a custom ORC-based JIT, KaleidoscopeJIT. This fixes a lot of the bugs
in Kaleidoscope that were introduced when we deleted the legacy JIT. The
documentation for Chapter 4, which introduces the JIT APIs, is updated to
reflect the change.

Also included are a number of C++11 modernizations and general cleanup. Where
appropriate, the docs have been updated to reflect these changes too.

llvm-svn: 246002