Support rvalue references in the gdb python module (includes doc/)

This patch adds the ability to inspect rvalue reference types and values using
the gdb python module. This is achieved by creating two wrappers for
valpy_reference_value(), using the ReferenceExplorer class to handle the
objects of rvalue reference types and placing necessary checks for a
TYPE_CODE_RVALUE_REF type code next to the checks for a TYPE_CODE_REF type
code.

gdb/ChangeLog

	PR gdb/14441
	* doc/python.texi (Types in Python): Add TYPE_CODE_RVALUE_REF to
	table of constants.
	* python/lib/gdb/command/explore.py: Support exploring values
	of rvalue reference types.
	* python/lib/gdb/types.py: Implement get_basic_type() for
	rvalue reference types.
	* python/py-type.c (pyty_codes) <TYPE_CODE_RVALUE_REF>: New
	constant.
	* python/py-value.c (valpy_getitem): Add an rvalue reference
	check.
	(valpy_reference_value): Add new parameter "refcode".
	(valpy_lvalue_reference_value, valpy_rvalue_reference_value):
	New wrappers for valpy_reference_value().
	* python/py-xmethods.c (gdbpy_get_xmethod_result_type)
	(gdbpy_invoke_xmethod): Likewise.

gdb/python/py-type.c

@@ -106,6 +106,7 @@ static struct pyty_code pyty_codes[] =
   ENTRY (TYPE_CODE_METHODPTR),
   ENTRY (TYPE_CODE_MEMBERPTR),
   ENTRY (TYPE_CODE_REF),
+  ENTRY (TYPE_CODE_RVALUE_REF),
   ENTRY (TYPE_CODE_CHAR),
   ENTRY (TYPE_CODE_BOOL),
   ENTRY (TYPE_CODE_COMPLEX),