Structures passed by value

[trombonehero]

LOOM should be able to instrument functions that accept structures by value or use sret. The details of how to do this may be ABI-dependant. Steps:

1. write tests for structure-by-value parameters and (separately) for sret,
2. explore ABI-dependence of Clang output and
3. teach LOOM how to deal with these details.

[trombonehero]

This is more generic a requirement than just FBT. In general, when we want to instrument a structure value, we should do something sensible with the fields inside it.

This might also have implications for how we do libxo things: we might want to open a new object to explicitly record those fields as belonging to a container rather than flattening them.