Add $_as_string convenience function

This patch is a follow-up to "Add printf format specifier for printing
enumerator":

  https://sourceware.org/ml/gdb-patches/2016-02/msg00144.html

Instead of having a solution specific to the printf command, Pedro
suggested adding a general purpose function $_as_string() that would
cover this use case and more.

So, in order to print the textual label of an enum, one can use:

  (gdb) printf "Visiting node of type %s\n", $_as_string(node)
  Visiting node of type NODE_INTEGER

gdb/ChangeLog:

	* data-directory/Makefile.in (PYTHON_FILE_LIST): Install
	gdb/function/as_string.py.
	* python/lib/gdb/function/as_string.py: New file.
	* NEWS: Mention the new $_as_string function.

gdb/testsuite/ChangeLog:

	* gdb.python/py-as-string.exp: New file.
	* gdb.python/py-as-string.c: New file.

gdb/doc/ChangeLog:

	* gdb.texinfo (Convenience Functions): Document $_as_string.

gdb/testsuite/gdb.python/py-as-string.c

@@ -0,0 +1,32 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2016 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/>.  */
+
+enum EnumType {
+  ENUM_VALUE_A,
+  ENUM_VALUE_B,
+  ENUM_VALUE_C,
+  ENUM_VALUE_D,
+};
+
+static enum EnumType enum_valid = ENUM_VALUE_B;
+static enum EnumType enum_invalid = 20;
+
+int
+main ()
+{
+  return 0;
+}