[RFC] Export valid llvm module with code_llvm

Hi,

when looking into possible optimizations for the LLVM-IR generated by Julia, it is generally useful to 
have code_llvm emit LLVM-IR that is loadable into LLVM's opt tool. The following two patches tweak the
LLVM-IR dumping to make this possible.

This is more a RFC than a finished pull request. The current code has only be tested with LLVM_VER=svn. If this functionality is desired, the changes still need to be tested with older versions of LLVM.