[yaml2obj] Add a `-E` flag to preprocess only.

If you're having trouble getting a yaml2obj macro expansion to do what
you want, it's useful to be able to print the output of the
preprocessing to see what your macros expanded to //before// going
into the YAML processing phase.

yaml2obj has its own preprocessing system which isn't the same as any
other well-known thing like cpp. So there's no way to do this macro
expansion via another tool: yaml2obj will have to do it itself.

In this commit I add an `-E` flag to yaml2obj to do that.

Differential Revision: https://reviews.llvm.org/D130981