internal/lsp: update diagnostic snapshotID even if not published

The LSP server tracks the snapshot ID for which diagnostics were last
published, but this ID was not being updated if the publish was
suppressed when there are no changes, which can cause incorrect
diagnostics when snapshots are published out-of-order (a separate bug).

Fix this to update the snapshot ID when the current published
diagnostics match the latest computed diagnostics.

Fixes golang/go#42837

Change-Id: I3634e6f351a479aefa5c9423956720431590d814
Reviewed-on: https://go-review.googlesource.com/c/tools/+/275238
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Robert Findley <rfindley@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/diagnostics.go

@@ -485,6 +485,9 @@ func (s *Server) publishDiagnostics(ctx context.Context, final bool, snapshot so
 		source.SortDiagnostics(diags)
 		hash := hashDiagnostics(diags...)
 		if hash == r.publishedHash {
+			// Update snapshotID to be the latest snapshot for which this diagnostic
+			// hash is valid.
+			r.snapshotID = snapshot.ID()
 			continue
 		}
 		version := float64(0)