cmd/gopls/integration/vscode: log diagnostics as they are received
cmd/gopls/integration/vscode: log diagnostics as they are received

This change will allow us to investigate golang/go#30786 more easily.
Diagnostics are getting stuck even though they appear to be being sent
in the correct order, so it will be helpeful to see what's happening on
the VSCode side.

Change-Id: I623fcd9979c05decb0a6f60da2f75af7d0ff3853
Reviewed-on: https://go-review.googlesource.com/c/tools/+/170895
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools