Flatten all interface variables (#2217)

Flatten all interface variables (#2217)

Specifically, add flattening of arrayed io for geometry and
tesselation shaders. Previously some interface structs just had
builtins split out which caused some interfaces to not be exactly
the same as that of flattened adjacent stages, affecting validation
and correctness.

This obviates builtin splitting. That will be removed in a followup
commit. It was left in for this commit to better exhibit the functional
changes that were made.

Fixes #1660.