Well, this was a journey!

The original issue I was trying to fix is covered by the new behavior
test in array.zig: in essence, `ty` and `coerced_ty` result locations
were not correctly propagated.

While fixing this, I noticed a similar bug in struct inits: the type was
propagated to *fields* fine, but the actual struct init was
unnecessarily anonymous, which could lead to unnecessary copies. Note
that the behavior test added in struct.zig was already passing - the bug
here didn't change any easy-to-test behavior - but I figured I'd add it
anyway.

This is a little harder than it seems, because the result type may not
itself be an array/struct type: it could be an optional / error union
wrapper. A new ZIR instruction is introduced to unwrap these.

This is also made a little tricky by the fact that it's possible for
result types to be unknown at the time of semantic analysis (due to
`anytype` parameters), leading to generic poison. In these cases, we
must essentially downgrade to an anonymous initialization.

Fixing these issues exposed *another* bug, related to type resolution in
Sema. That issue is now tracked by #16603. As a temporary workaround for
this bug, a few result locations for builtin function operands have been
disabled in AstGen. This is technically a breaking change, but it's very
minor: I doubt it'll cause any breakage in the wild.