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 )

2 files changed, 77 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.fortran/namelist.exp b/gdb/testsuite/gdb.fortran/namelist.exp
new file mode 100644
index 0000000..d6263e1
--- /dev/null
+++ b/gdb/testsuite/gdb.fortran/namelist.exp
@@ -0,0 +1,50 @@
+# Copyright (C) 2021-2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the gdb testsuite.  It contains tests for fortran
+# namelist.
+
+if { [skip_fortran_tests] } { return -1 }
+
+standard_testfile .f90
+load_lib "fortran.exp"
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug f90}]} {
+    return -1
+}
+
+if ![fortran_runto_main] then {
+    perror "couldn't run to main"
+    continue
+}
+
+# Depending on the compiler being used, the type names can be printed
+# differently.
+set int [fortran_int4]
+
+gdb_breakpoint [gdb_get_line_number "Display namelist"]
+gdb_continue_to_breakpoint "Display namelist"
+
+if {[test_compiler_info {gcc-*}]} {
+    gdb_test "ptype nml" \
+        "type = Type nml\r\n *$int :: a\r\n *$int :: b\r\n *End Type nml"
+    gdb_test "print nml" \
+        "\\$\[0-9\]+ = \\( a = 10, b = 20 \\)"
+} else {
+    gdb_test "ptype nml" \
+        "No symbol \"nml\" in current context\\."
+    gdb_test "print nml" \
+        "No symbol \"nml\" in current context\\."
+}
diff --git a/gdb/testsuite/gdb.fortran/namelist.f90 b/gdb/testsuite/gdb.fortran/namelist.f90
new file mode 100644
index 0000000..9e2ba04
--- /dev/null
+++ b/gdb/testsuite/gdb.fortran/namelist.f90
@@ -0,0 +1,27 @@
+! Copyright (C) 2021-2022 Free Software Foundation, Inc.
+!
+! This program is free software; you can redistribute it and/or modify
+! it under the terms of the GNU General Public License as published by
+! the Free Software Foundation; either version 3 of the License, or
+! (at your option) any later version.
+!
+! This program is distributed in the hope that it will be useful,
+! but WITHOUT ANY WARRANTY; without even the implied warranty of
+! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+! GNU General Public License for more details.
+!
+! You should have received a copy of the GNU General Public License
+! along with this program.  If not, see <http://www.gnu.org/licenses/>.
+!
+! This file is the Fortran source file for namelist.exp.
+
+program main
+
+  integer :: a, b
+  namelist /nml/ a, b
+
+  a = 10
+  b = 20
+  Write(*,nml) ! Display namelist
+
+end program main