global const vs. const tuple assignment inconsistency

`const (c,d) = (3,4)` works but `global const (a,b) = (3,4)` does not.  Seems wrong to me.

Probably related to #7314, but I thought using parens was suppose to work here.

It is desirable to have a syntax for assignment of a tuple to `global const` values, if you want to initialize several constants from a function returning a tuple.
