Support for MCJIT
* Link against MCJIT libraries from LLVM
* Amend LLVM_Util to have a debug mode.
* Some incidental movement of BackendLLVM methods from llvm_instance.cpp
  to backendllvm.cpp.
* Some minor changes to LLVM_Util to support MCJIT.
* Moved some non-essential (and unlikely to inline) functions from
  llvm_opt.cpp to other cpp files. This helps reduce the dependencies
  for llvm_ops.cpp, reduce the amount of bitcode to process, and was
  intended to fix some MCJIT issues.
* Various fixes to make it all work, MCJIT seems a little fragile about
  predigested bitcode.

Currently, this code uses ParseBitcodeFile rather than getLazyBitcodeModule
when MCJIT is enabled, since MCJIT appears to have a bug that causes it to
not correctly find the symbols in the module when using lazy deserialization.