Debugging support for codegen'd code

After the switch to MCJIT and LLVM 3.7.0 (IMPALA-775) it should be possible to debug codegened code, since MCJIT can emit DWARF debug info and gdb versions 7 and higher support finding the debug info for LLVM's jitted code. (Ref http://llvm.org/docs/DebuggingJITedCode.html) There are a few things to do to make sure that this works well:

• Get LLVM to emit cross-compiled code with debug symbols for the debug build
• Work out how to add debug symbols to IRBuilder-generated code (maybe with the DebugIR LLVM pass).
• Test that it actually works