[flang] Allow array constructor implied DO loop indices as constant expressions

[flang] Allow array constructor implied DO loop indices as constant expressions

When the bounds of an implied DO loop in an array constructor are
constant, the index variable of that loop is considered a constant
expression and can be used as such in the items in the value list
of the implied DO loop.  Since the KIND type parameter values of items
in the value list can depend on the various values taken by such an
index, it is not possible to represent those values with a single
typed expression.  So implement such loops by taking multiple passes
over the parse tree of the implied DO loop instead.

Differential revision: https://reviews.llvm.org/D90494