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, 13 insertions, 0 deletions

diff --git a/gdb/gdbtypes.h b/gdb/gdbtypes.h
index 7238873e4db..5072dc24bfa 100644
--- a/gdb/gdbtypes.h
+++ b/gdb/gdbtypes.h
@@ -196,6 +196,19 @@ enum type_code
 
     /* * Fixed Point type.  */
     TYPE_CODE_FIXED_POINT,
+
+    /* * Fortran namelist is a group of variables or arrays that can be
+       read or written.
+
+       Namelist syntax: NAMELIST / groupname / namelist_items ...
+       NAMELIST statement assign a group name to a collection of variables
+       called as namelist items. The namelist items can be of any data type
+       and can be variables or arrays.
+
+       Compiler emit DW_TAG_namelist for group name and DW_TAG_namelist_item
+       for each of the namelist items. GDB process these namelist dies
+       and print namelist variables during print and ptype commands.  */
+    TYPE_CODE_NAMELIST,
   };
 
 /* * Some bits for the type's instance_flags word.  See the macros