libstdc++: Normalise _GLIBCXX20_DEPRECATED macro
Updates _GLIBCXX20_DEPRECATED to be defined and behave the same as the
versions for other standards (e.g. _GLIBCXX17_DEPRECATED).
libstdc++-v3/ChangeLog:
* doc/doxygen/user.cfg.in (PREDEFINED): Update macros.
* include/bits/c++config (_GLIBCXX20_DEPRECATED): Make
consistent with other 'deprecated' macros.
* include/std/type_traits (is_pod, is_pod_v): Use
_GLIBCXX20_DEPRECATED_SUGGEST instead.
Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>