rollup merge of #19413: P1start/more-trailing-commas

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.

diff --cc src/libsyntax/parse/attr.rs
Simple merge

diff --cc src/libsyntax/parse/parser.rs
Simple merge