Fix bug in Obj-C type encoding for structures.

 - Mechanism for detecting if a structure should be expanded wasn't
   reliable. Simplified by just keeping track of what we should be
   expanding.

 - This fixes a bug in using NSInvocation to invoke a method which
   returned a structure, which in used by Key Value Observing, which
   in the end, caused a miscompile in poor little Sketch.

llvm-svn: 57675