c++: Fix pasto in structured binding diagnostics [PR94571]

This snippet has been copied from the non-structured binding declaration
parsing later in the function, and while for non-structured bindings
it can be followed by comma or semicolon, structured bindings may be
only followed by semicolon.

Or, do we want to have a different message for the case when there is
a comma (and keep this corrected one only if there is something else)
that would explain better what is the bug (or add a fix-it hint)?
Marek said in the PR that clang++ reports
error: decomposition declaration must be the only declaration in its group

There is another thing Marek noted (though, something for different spot),
that diagnostic for auto x(1), [e,f] = test2; could also use a clearer
wording like the above (or a fix-it hint), but the question is if we should
assume [ after , as a structured binding or if we should do some tentative
parsing first to figure out if it looks like a structured binding.

2020-04-16  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94571
	* parser.c (cp_parser_simple_declaration): Fix up a pasto in
	diagnostics.

	* g++.dg/cpp1z/decomp51.C: New test.