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"
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