[flang] Fix NAMELIST input bug with multiple subscript triplets

[klausler]

Summary

NAMELIST input can contain array subscripts with triplet notation.
The calculation of the default effective stride for the constructed
array descriptor was simply incorrect after the first dimension.

flang/runtime/namelist.cpp

 //===-- runtime/namelist.cpp ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "namelist.h"
 #include "descriptor-io.h"
 #include "io-stmt.h"
 #include "flang/Runtime/io-api.h"
+#include <algorithm>
 #include <cstring>
 #include <limits>
 
 namespace Fortran::runtime::io {
 
 // Max size of a group, symbol or component identifier that can appear in
 // NAMELIST input, plus a byte for NUL termination.
 static constexpr std::size_t nameBufferSize{201};

 static bool HandleSubscripts(IoStatementState &io, Descriptor &desc,
     const Descriptor &source, const char *name) {
   IoErrorHandler &handler{io.GetIoErrorHandler()};
   io.HandleRelativePosition(1); // skip '('
   // Allow for blanks in subscripts; they're nonstandard, but not
   // ambiguous within the parentheses.
   SubscriptValue lower[maxRank], upper[maxRank], stride[maxRank];
   int j{0};
-  std::size_t elemLen{source.ElementBytes()};
+  std::size_t contiguousStride{source.ElementBytes()};
   bool ok{true};
   std::optional<char32_t> ch{io.GetNextNonBlank()};
   for (; ch && *ch != ')'; ++j) {
     SubscriptValue dimLower{0}, dimUpper{0}, dimStride{0};
     if (j < maxRank && j < source.rank()) {
       const Dimension &dim{source.GetDimension(j)};
       dimLower = dim.LowerBound();
       dimUpper = dim.UpperBound();
-      dimStride = elemLen ? dim.ByteStride() / elemLen : 1;
+      dimStride =
+          dim.ByteStride() / std::max<SubscriptValue>(contiguousStride, 1);
+      contiguousStride *= dim.Extent();
     } else if (ok) {
       handler.SignalError(
           "Too many subscripts for rank-%d NAMELIST group item '%s'",
           source.rank(), name);
       ok = false;
     }
     if (auto low{GetSubscriptValue(io)}) {
       if (*low < dimLower || (dimUpper >= dimLower && *low > dimUpper)) {