When a label in a WGSL front end error has an undefined span, omit the
label from the error message. This is not great, but because of the
way Naga IR represents local variable references it is hard to get the
right span, and omitting the label better than panicking in `unwrap`,
since the error message has a general message anyway.
When reporting errors in construction expressions, use the span of the
constructor itself (that is, the type name) in preference to the span
of the overall expression. This makes errors easier to follow.
When a constructor builtin has an explicit type parameter, like
`mat2x2<f32>`, it should not produce an abstract matrix, even if its
arguments are abstract.
