[flang] Adjust semantics of the char length of an array constructor (#97337)

An implied DO loop with no trips in an array constructor does not have a
well-defined character length unless its data items have a length that
is constant expression. That works, but the implementation is too
broadly applied. An array constructor with an explicit type-spec always
has a well-defined length.

flang/lib/Semantics/expression.cpp

@@ -1584,7 +1584,8 @@ private:
   std::optional<Expr<SubscriptInteger>> LengthIfGood() const {
     if (type_) {
       auto len{type_->LEN()};
-      if (len && IsConstantExpr(*len) && !ContainsAnyImpliedDoIndex(*len)) {
+      if (explicitType_ ||
+          (len && IsConstantExpr(*len) && !ContainsAnyImpliedDoIndex(*len))) {
         return len;
       }
     }