Add ability to return rvalue reference values from value_ref

Add ability to return rvalue reference values from value_ref

Parameterize value_ref() by the kind of reference type the value of which
is requested. Change all callers to use the new API.

gdb/ChangeLog

	PR gdb/14441
	* ada-lang.c (ada_evaluate_subexp): Adhere to the new
	value_ref() interface.
	* c-valprint.c (c_value_print): Likewise.
	* infcall.c (value_arg_coerce): Likewise.
	* python/py-value.c (valpy_reference_value): Likewise.
	* valops.c (value_cast, value_reinterpret_cast)
	(value_dynamic_cast, typecmp): Likewise.
	(value_ref): Parameterize by kind of return value reference type.
	* value.h (value_ref): Add new parameter "refcode".