Figure out default construction and brace-initialization of C++ structs

[Manishearth]

See unicode-org/icu4x#2522

This is related to #247

These structs have default ctors; which create uninitialized memory. We shouldn't do that, and we should ask people to = {} instead.

We probably want to delete the default ctor; however it's also necessary for brace initialization (Foo foo = {.bar = ..., .baz = ...}).

Ideally we can hook this into #247 so that a default ctor is generated when all children have a default value.

[Manishearth]

cc @sffc who has been working on the C++ backend