gdb, dwarf: Don't follow the parent of a subprogram to get a prefix.

During prefix resolution, if the parent is a subprogram, there is no need
to go to the parent of the subprogram.  The DIE will be local.

For a program like:
~~~
  class F1
  {
  public:
    int a;
    int
    vvv ()
    {
      class F2
      {
	int f;
      };
      F2 abcd;
      return 1;
    }
  };
~~~

The class F2 should not be seen as a member of F1.

Before:
~~~
(gdb) ptype abcd
type = class F1::F2 {
  private:
    int f;
}
~~~

After:
~~~
(gdb) ptype abcd
type = class F2 {
  private:
    int f;
}
~~~

gdb/ChangeLog:
2021-06-23  Felix Willgerodt  <felix.willgerodt@intel.com>

	* dwarf2/read.c (determine_prefix): Return an empty prefix if the
	parent is a subprogram.

gdb/testsuite/ChangeLog:
2021-06-23  Felix Willgerodt  <felix.willgerodt@intel.com>

	* gdb.cp/nested-class-func-class.cc: New file.
	* gdb.cp/nested-class-func-class.exp: New file.

gdb/dwarf2/read.c

@@ -22657,7 +22657,7 @@ determine_prefix (struct die_info *die, struct dwarf2_cu *cu)
 		&& (dwarf2_name (parent, cu) != NULL))
 	      return dwarf2_name (parent, cu);
 	  }
-	return determine_prefix (parent, cu);
+	return "";
       case DW_TAG_enumeration_type:
 	parent_type = read_type_die (parent, cu);
 	if (parent_type->is_declared_class ())

gdb/testsuite/gdb.cp/nested-class-func-class.cc

@@ -0,0 +1,46 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2021 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/>.  */
+
+int
+foo ()
+{
+  class F1
+  {
+  public:
+    int a;
+    int
+    vvv ()
+    {
+      class F2
+      {
+	int f;
+      };
+      F2 abcd;
+      return 1; /* BP 1 */
+    }
+  };
+
+  F1 x;
+  int a = x.vvv ();
+  return a; /* BP 2 */
+}
+
+int
+main ()
+{
+  return foo ();
+}

gdb/testsuite/gdb.cp/nested-class-func-class.exp

@@ -0,0 +1,47 @@
+# Copyright 2021 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/>.
+
+# Regression test for type printing of private nested classes.
+
+if {[skip_cplus_tests]} { continue }
+
+load_lib "cp-support.exp"
+
+standard_testfile .cc
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug c++}]} {
+    return -1
+}
+
+if {![runto_main]} {
+    return -1
+}
+
+gdb_breakpoint [gdb_get_line_number "BP 1"]
+gdb_breakpoint [gdb_get_line_number "BP 2"]
+gdb_continue_to_breakpoint "BP 1"
+
+cp_test_ptype_class \
+    "F2" "" "class" "F2" \
+    {
+	{ field private "int f;" }
+    }
+
+gdb_continue_to_breakpoint "BP 2"
+
+with_test_prefix "BP 2" {
+    gdb_test "ptype F1::F2" "There is no field named F2"
+    gdb_test "ptype F2" "No symbol \"F2\" in current context."
+}