MS ABI: Add an additional test for empty structs in C

Empty structs in C differ from those in C++.
- C++ requires that empty types have size 1; alignment requirements may
  increase the size of the struct.
- The C implementation doesn't let empty structs have a size under 4
  bytes.  Again, alignment requirements may increase the struct's size.

Add a test to stress these differences.

llvm-svn: 218963