x/tools/gopls: add the unusedwrite analysis

[ainar-g]

CL 287034 added the new unusedwrite analysis pass. I think it would be nice to have in gopls.

I'm not sure if it should be among the default analysers, but either way, if there are no objections to this, I am ready to send a CL.

[stamblerre]

We'd be happy to accept a CL--if you think it should be off by default, we can start with that first, and then enable it later on?

[stamblerre]

Ah, looks like it uses SSA, so let's keep it off by default to start.

[gopherbot]

Change https://golang.org/cl/295172 mentions this issue: internal/lsp/source: add the unusedwrite analyzer

[ainar-g]

Thanks for reviewing!