Sketch out an CFG reconstruction mode for llvm-objdump.

- Not great yet, but it's a start.
- Requires an object file with a symbol table. (I really want to fix this, but it'll need a whole new algorithm)
- ELF and COFF won't work at the moment due to libObject shortcomings.

To try it out run
$ llvm-objdump -d --cfg foo.o

This will create a graphviz file for every symbol in the object file's text section containing a CFG.

llvm-svn: 135608