gopls/internal/lsp/work: use the WorkFileError diagnostics source
gopls/internal/lsp/work: use the WorkFileError diagnostics source

It looks like this source was added but never used. Use it.

Change-Id: I19d7c49ab1fda11d078a7850a7c0c3a78133ff52
Reviewed-on: https://go-review.googlesource.com/c/tools/+/435361
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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