Implement WG21 P2156R1/WG14 N2557 on duplicate attributes

These proposals make the same changes to both C++ and C and remove a
restriction on standard attributes appearing multiple times in the same
attribute list.

We could warn on the duplicate attributes, but do not. This is for
consistency as we do not warn on attributes duplicated within the
attribute specifier sequence. If we want to warn on duplicated
standard attributes, we should do so both for both situations:
[[foo, foo]] and [[foo]][[foo]].