Ensure that compound literals with array type have checked type in checked scopes.

[dtarditi]

A compound literal with array type has the form (T[n]) { e1, e2, e3, ... }. We need to make sure that the array type ends up being checked in a checked scope.

We can either require (1) that the array type be completely checked or (2) convert any unchecked array type to the corresponding checked array type and verify that the resulting type is completely checked. I am leaning toward the second option because it'll be less typing for programmers.

[dtarditi]

Completed by PR #396.