LLJIT is a prefabricated ORC based JIT class that is meant to be the go-to
replacement for MCJIT. Unlike OrcMCJITReplacement (which will continue to be
supported) it is not API or bug-for-bug compatible, but targets the same
use cases: Simple, non-lazy compilation and execution of LLVM IR.

LLLazyJIT extends LLJIT with support for function-at-a-time lazy compilation,
similar to what was provided by LLVM's original (now long deprecated) JIT APIs.

This commit also contains some simple utility classes (CtorDtorRunner2,
LocalCXXRuntimeOverrides2, JITTargetMachineBuilder) to support LLJIT and
LLLazyJIT.

Both of these classes are works in progress. Feedback from JIT clients is very
welcome!

llvm-svn: 335670