[flang] Runtime: Reset list-directed input state for each NAMELIST item

[klausler]

Summary

NAMELIST I/O formatting uses the runtime infrastructure for
list-directed I/O. List-directed input processing has same state
that requires reinitialization for each successive NAMELIST input
item. This patch fixes bugs with "null" items and repetition counts
on NAMELIST input items after the first in the group.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/runtime/io-stmt.h

   std::optional<DataEdit> GetNextDataEdit(
       IoStatementState &, int maxRepeat = 1);
 
+  // Each NAMELIST input item is a distinct "list-directed"
+  // input statement.  This member function resets this state
+  // so that repetition and null values work correctly for each
+  // successive NAMELIST input item.
+  void ResetForNextNamelistItem() {
+    remaining_ = 0;
+    isFirstItem_ = true;
+    realPart_ = imaginaryPart_ = false;
+  }
+
 private:
   int remaining_{0}; // for "r*" repetition
   std::int64_t initialRecordNumber_;

flang/runtime/namelist.cpp

   ConnectionState &connection{io.GetConnectionState()};
   connection.modes.inNamelist = true;
   IoErrorHandler &handler{io.GetIoErrorHandler()};
+  auto *listInput{io.get_if<ListDirectedStatementState<Direction::Input>>()};
+  RUNTIME_CHECK(handler, listInput != nullptr);
   // Check the group header
   std::optional<char32_t> next{io.GetNextNonBlank()};
   if (!next || *next != '&') {

     }
     io.HandleRelativePosition(1);
     // Read the values into the descriptor
+    listInput->ResetForNextNamelistItem();
     if (!descr::DescriptorIO<Direction::Input>(io, *useDescriptor)) {
       return false;
     }