Forbid constructing tuple-related types via "new"

[gafter]

We decided for forbid creating a tuple type using the tuple syntax (#10891) so that we can reserve the syntax new (1, 2) for a target-typed object creation expression (#35). However there are related syntax forms that we may want to forbid as well.

new (Int32, Int32)[] { (1, 2), (1, 2) }
new (Int32, Int32)?()
new (Int32, Int32)?[] { (1, 2), null }
new (Int32, Int32)?[10]

Forbidding these syntax forms today will make integration of #35 easier later.

[jcouv]

Closing based on 2016-10-18 LDM notes:

Use of tuples in new

#14216
We should certainly allow arrays of tuples to be created. Probably fine to also keep allowing ?.

[VSadov]

@jcouv Thanks!!