c, c++: -Wswitch warning on [[maybe_unused]] enumerator [PR105497]

c, c++: -Wswitch warning on [[maybe_unused]] enumerator [PR105497]

This PR complains that we emit the "enumeration value not handled in
switch" warning even though the enumerator was marked with the
[[maybe_unused]] attribute.

I couldn't just check TREE_USED, because the enumerator could have been
used earlier in the function, which doesn't play well with the
c_do_switch_warnings warning.  Instead, I had to check the attributes on
the CONST_DECL.  This is easy since the TYPE_VALUES of an enum type are
now consistent between C and C++, both of which store the CONST_DECL in
its TREE_VALUE.

	PR c++/105497

gcc/c-family/ChangeLog:

	* c-warn.cc (c_do_switch_warnings): Don't warn about unhandled
	enumerator when it was marked with attribute unused.

gcc/testsuite/ChangeLog:

	* c-c++-common/Wswitch-1.c: New test.
	* g++.dg/warn/Wswitch-4.C: New test.