[wgsl-in] Unify ConcreteConstructor and ConcreteConstructorHandle.

Replace the `ConcreteConstructor` and `ConcreteConstructorHandle`
types in `front::wgsl::lower::construction` with a single type
`Constructor` with a type parameter that determines how it refers to
Naga types.

In the single-argument vector construction case, construct `Compose`
expressions when the lengths don't match, and leave the problem for
validation to catch; `validate_compose` is a better place to invest
time in improving diagnostics (and arguably producess more specific
error messages already).