Require commas between double square bracket attributes.

Clang currently has a bug where it allows you to write [[foo bar]] and
both attributes are silently accepted. This patch corrects the comma
parsing rules for such attributes and handles the test case fallout, as
a few tests were accidentally doing this.

GitOrigin-RevId: 5ad15f4d1c6f56d25904265023d123a7d0b9d59d
8 files changed