Add definitions for rvalue reference types

Add definitions for rvalue reference types

This patch introduces preliminal definitions regarding C++11 rvalue references
to the gdb type system. In addition to an enum type_code entry, a field in
struct type and an accessor macro for that which are created similarly to the
lvalue references counterparts, we also introduce a TYPE_REFERENCE convenience
macro used to check for both kinds of references simultaneously as they are
equivalent in many contexts.

gdb/Changelog

    PR gdb/14441
    * gdbtypes.h (enum type_code) <TYPE_CODE_RVALUE_REF>: New constant.
    (TYPE_IS_REFERENCE): New macro.
    (struct type): Add rvalue_reference_type field.
    (TYPE_RVALUE_REFERENCE_TYPE): New macro.