Allow trailing commas in array patterns and attributes
auto merge of #19413 : P1start/rust/more-trailing-commas, r=alexcrichton

The only other place I know of that doesn’t allow trailing commas is closure types (#19414), and those are a bit tricky to fix (I suspect it might be impossible without infinite lookahead) so I didn’t implement that in this patch. There are other issues surrounding closure type parsing anyway, in particular #19410.