gdb/fortran: support ptype and print commands for namelist variables

Gfortran supports namelists (a Fortran feature); it emits
DW_TAG_namelist and DW_TAG_namelist_item dies. But gdb does not
process these dies and does not support 'print' or 'ptype' commands on
namelist variables.

An attempt to print namelist variables results in gdb bailing out with
the error message as shown below.

  (gdb) print nml
  No symbol "nml" in current context.

This commit is to make the print and ptype commands work for namelist
variables and its items. Sample output of these commands is shared
below, with fixed gdb.

  (gdb) ptype nml
  type = Type nml
      integer(kind=4) :: a
      integer(kind=4) :: b
  End Type nml
  (gdb) print nml
  $1 = ( a = 10, b = 20 )

1 files changed, 20 insertions, 6 deletions

diff --git a/gdb/f-valprint.c b/gdb/f-valprint.c
index e8d8627..6a199f1 100644
--- a/gdb/f-valprint.c
+++ b/gdb/f-valprint.c
@@ -512,24 +512,38 @@ f_language::value_print_inner (struct value *val, struct ui_file *stream,
 
     case TYPE_CODE_STRUCT:
     case TYPE_CODE_UNION:
+    case TYPE_CODE_NAMELIST:
       /* Starting from the Fortran 90 standard, Fortran supports derived
 	 types.  */
       fprintf_filtered (stream, "( ");
       for (index = 0; index < type->num_fields (); index++)
 	{
-	  struct value *field = value_field (val, index);
-
-	  struct type *field_type = check_typedef (type->field (index).type ());
-
+	  struct type *field_type
+	    = check_typedef (type->field (index).type ());
 
 	  if (field_type->code () != TYPE_CODE_FUNC)
 	    {
-	      const char *field_name;
+	      const char *field_name = type->field (index).name ();
+	      struct value *field;
+
+	      if (type->code () == TYPE_CODE_NAMELIST)
+		{
+		  /* While printing namelist items, fetch the appropriate
+		     value field before printing its value.  */
+		  struct block_symbol sym
+		    = lookup_symbol (field_name, get_selected_block (nullptr),
+				     VAR_DOMAIN, nullptr);
+		  if (sym.symbol == nullptr)
+		    error (_("failed to find symbol for name list component %s"),
+			   field_name);
+		  field = value_of_variable (sym.symbol, sym.block);
+		}
+	      else
+		field = value_field (val, index);
 
 	      if (printed_field > 0)
 		fputs_filtered (", ", stream);
 
-	      field_name = type->field (index).name ();
 	      if (field_name != NULL)
 		{
 		  fputs_styled (field_name, variable_name_style.style (),