Prohibit stackalloc int[3][1]

[jskeet]

In #1211 it looks like stackalloc int[3][1] should be valid by the standard, but Roslyn complains. We've decided it wouldn't do what is probably expected anyway, so it would be better to prohibit it in the standard.

We haven't yet decided on the best approach for this (e.g. in the grammar or as an additional semantic rule in the text).

[gafter]

Possible strategy: Categorize it as an array creation expression in the grammar.

[jnm2]

Would it help those who come after us to say something explicit, so that people don't have to try to tease apart whether or not things were blocked by design and what the rationale might have been?

[gafter]

@jnm2 Yes, and the proposed solution piggy-backs on such text already in the standard:

Primary expressions are divided between array_creation_expressions and primary_no_array_creation_expressions. Treating array_creation_expression in this way, rather than listing it along with the other simple expression forms, enables the grammar to disallow potentially confusing code such as

object o = new int[3][1];

which would otherwise be interpreted as

object o = (new int[3])[1];