Cope with empty structs in function calls

A function that takes a struct without any contents—whether anonymous or named—will cause the compile to assert out if referred:

```
$ ../terra structempty.t
terra: src/tcompiler.cpp:562: llvm::Type *CCallingConv::TypeForClass(size_t, CCallingConv::RegisterClass): Assertion `!"unexpected class"' failed.
[1]    22805 abort (core dumped)  ../terra structempty.t
$
```