HLSL: rewrite handling of interface matching rules (#1276)

* [hlsl-out] flatten the entry point inputs

Previously, the logic was re-ordering the inputs according to the binding.
This breaks if one of the inputs is a struct. With this change, the struct
fields are also flattened into the fake entry point struct. We also
construct the original arguments at the beginning of the function.

* hlsl-out: completely separate the flattened IO structs from the original IR structs

Previously, we had heuristics to detect if a particular struct needs the fields
to be re-ordered. We'd re-order interface structs without layout, and the detection
was very fragile and easily wrong.
The new logic is spawning separate struct types if we need any re-ordering to happen.
It's solid, there are no heuristics.