Fix missing location decoration for structures put directly on input/output interfaces

Spec for decorating the OpVariable:
	"The remaining variables listed by OpEntryPoint with the Input or Output storage class form the user-defined variable interface. These variables must be identified with a Location decoration"

Spec for decorating struct type:
	"The layout of a structure type used as an Input or Output depends on whether it is also a Block (i.e. has a Block decoration).
	If it is a not a Block, then the structure type must have a Location decoration"