[Python] Add methods reference_value and const_value to gdb.Value.

[Python] Add methods reference_value and const_value to gdb.Value.

gdb/ChangeLog:

	* NEWS (Python Scripting): Mention the new gdb.Value methods.
	* python/py-value.c (valpy_reference_value): New function.
	(valpy_const_value): Likewise.
	(value_object_methods): Add new methods.
	* value.c (make_cv_value): New function.
	* value.h (make_cv_value): Declare.

gdb/doc/ChangeLog:

	* python.texi (Values From Inferior): Add descriptions of new
	methods gdb.Value.reference_value and gdb.Value.const_value.

gdb/testsuite/ChangeLog:

	* gdb.python/py-xmethods.cc: Enhance test case.
	* gdb.python/py-xmethods.exp: New tests.
	* gdb.python/py-xmethods.py (A_indexoper): New xmethod worker
	function.
	(B_indexoper): Likewise.
	(global_dm_list) : Add new xmethod worker functions.