go/types: don't drop type in n:1 var decl if one is given
In n:1 variable declarations (multiple lhs variables with single
multi-valued initialization expression) where also a variable
type is provided, make sure that that type is assigned to all
variables on the lhs before the init expression assignment is
checked. Otherwise, (some) variables are assumed to take the type
of the corresponding value of the multi-valued init expression.
Fixes #15755.
Change-Id: I969cb5a95c85e28dbb38abd7fa7df16ff5554c03
Reviewed-on: https://go-review.googlesource.com/23313 Reviewed-by: Alan Donovan <adonovan@google.com>