Clarify scalar constructor with non-scalar values.

Constructor section of the spec says:

> Scalar constructors with non-scalar parameters can be used to take the first element from a non-scalar.

It does not appear to list _which_ non-scalar types are allowed. From testing in GLSLang it seems that `vec` and `mat` types can be used, but `array` and `struct` types cannot.

Should the spec be clarified to say `vector or matrix` instead of just `non-scalar` ?

Start with the Constructor section of the GLSL specification and compare its wording with the observed GLSLang behavior for vector, matrix, array, and struct arguments. Confirm which non-scalar types are permitted, then clarify the specification so the allowed types and first-element behavior are explicit.

Written by the indexing model from the issue text.