[Orc] New JIT APIs.

This patch adds a new set of JIT APIs to LLVM. The aim of these new APIs is to
cleanly support a wider range of JIT use cases in LLVM, and encourage the
development and contribution of re-usable infrastructure for LLVM JIT use-cases.

These APIs are intended to live alongside the MCJIT APIs, and should not affect
existing clients.

Included in this patch:

1) New headers in include/llvm/ExecutionEngine/Orc that provide a set of
   components for building JIT infrastructure.
   Implementation code for these headers lives in lib/ExecutionEngine/Orc.

2) A prototype re-implementation of MCJIT (OrcMCJITReplacement) built out of the
   new components.

3) Minor changes to RTDyldMemoryManager needed to support the new components.
   These changes should not impact existing clients.

4) A new flag for lli, -use-orcmcjit, which will cause lli to use the
   OrcMCJITReplacement class as its underlying execution engine, rather than
   MCJIT itself.

Tests to follow shortly.

Special thanks to Michael Ilseman, Pete Cooper, David Blaikie, Eric Christopher,
Justin Bogner, and Jim Grosbach for extensive feedback and discussion.

llvm-svn: 226940