c: Allow duplicate C2x standard attributes

c: Allow duplicate C2x standard attributes

N2557, accepted into C2x at the October WG14 meeting, removes the
requirement that duplicates of standard attributes cannot appear
within an attribute list (so allowing e.g. [[deprecated, deprecated]],
where previously that was disallowed but [[deprecated]] [[deprecated]]
was OK).  Remove the code checking for this (standard attributes
aren't in any released version of the C standard) and update tests
accordingly.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/c/
2020-10-27  Joseph Myers  <joseph@codesourcery.com>

	* c-parser.c (c_parser_std_attribute_specifier): Allow duplicate
	standard attributes.

gcc/testsuite/
2020-10-27  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/c2x-attr-deprecated-4.c, gcc.dg/c2x-attr-fallthrough-4.c,
	gcc.dg/c2x-attr-maybe_unused-4.c: Allow duplicate attributes.