Add the ability to dump the complete preprocessed C/C++ bindgen takes as input to a file

[fitzgen]

clang++ -save-temps doesn't seem to include any extra header files passed in as -include <header> which hurts debugging and reproducing issues in a standalone way. We use that to support multiple input header files. We should figure out how to work around this issue so we can accept multiple input headers and also get nice standalone debugging.

Perhaps create a synthetic in-memory file that has a #include <header> for every input header, and then we actually process that?

Not sure how to get libclang to dump a preprocessed file either.

[fitzgen]

This would help people who are filing issues with us file better issues, too.