[spv-out] Creating SPIR-V types is not, in fact, fallible.

There's no need for functions that construct SPIR-V types to be fallible, or for
their callers to check for errors.

As a consequence of this change, there may be other functions that no longer
need to be fallible, but Rust doesn't warn and Clippy doesn't lint, so we'll
have to address them as they come up.