Don't reuse the same diagnostic group id across buffers

This lets us use the group id as the key for an `ElementState`, which
fixes a panic that would occur in project diagnostics when opening it
while there were multiple diagnostic groups with the same id.
6baf8b033bAntonio Scandurra committed on 6/2/2022, 10:05:28 AMparentc34b30e
2 files changedLine totals unavailable