This looks like it was never properly implemented, since when we generate the
memberwise initializer for the struct in SILGen, it incorrectly tries to apply
the entire initializer expression to each variable binding in the pattern,
rather than destructuring the result and pattern-matching it to the variables.
Since it never worked it doesn't look like anyone is using this, so let's
put up an error saying it's unsupported until we can implement it properly.
Add `StructLetDestructuring` as an experimental feature flag so that tests around
the feature for things like module interface printing can still work.
