[flang][runtime] Skip unused truncated list-directed character input (#118320)

When reading non-delimited list-directed character input, read the whole
field even if it doesn't fit into the variable.

Fixes #118277.

Author: klausler

flang/runtime/edit-input.cpp

@@ -976,14 +976,10 @@ static RT_API_ATTRS bool EditListDirectedCharacterInput(
     return false;
   }
   // Undelimited list-directed character input: stop at a value separator
-  // or the end of the current record.  Subtlety: the "remaining" count
-  // here is a dummy that's used to avoid the interpretation of separators
-  // in NextInField.
-  Fortran::common::optional<int> remaining{length > 0 ? maxUTF8Bytes : 0};
-  while (Fortran::common::optional<char32_t> next{
-      io.NextInField(remaining, edit)}) {
+  // or the end of the current record.
+  while (auto ch{io.GetCurrentChar(byteCount)}) {
     bool isSep{false};
-    switch (*next) {
+    switch (*ch) {
     case ' ':
     case '\t':
     case '/':
@@ -1003,11 +999,17 @@ static RT_API_ATTRS bool EditListDirectedCharacterInput(
       break;
     }
     if (isSep) {
-      remaining = 0;
-    } else {
-      *x++ = *next;
-      remaining = --length > 0 ? maxUTF8Bytes : 0;
+      break;
+    }
+    if (length > 0) {
+      *x++ = *ch;
+      --length;
+    } else if (edit.IsNamelist()) {
+      // GNU compatibility
+      break;
     }
+    io.HandleRelativePosition(byteCount);
+    io.GotChar(byteCount);
   }
   Fortran::runtime::fill_n(x, length, ' ');
   return true;