fix(lsp): store result id for unchanged diagnostic reports

**Problem:** For unchanged document diagnostic reports, the `resultId`
is ignored completely, even though it should still be saved for the
request (in fact, the spec marks it as mandatory for unchanged reports,
so it should be extra important).

**Solution:** Always store the `resultId`.

runtime/lua/vim/lsp/diagnostic.lua

@@ -269,16 +269,20 @@ function M.on_diagnostic(error, result, ctx)
     return
   end
 
-  if result == nil or result.kind == 'unchanged' then
+  if result == nil then
     return
   end
 
   local client_id = ctx.client_id
-  handle_diagnostics(ctx.params.textDocument.uri, client_id, result.items, true)
-
   local bufnr = assert(ctx.bufnr)
   local bufstate = bufstates[bufnr]
   bufstate.client_result_id[client_id] = result.resultId
+
+  if result.kind == 'unchanged' then
+    return
+  end
+
+  handle_diagnostics(ctx.params.textDocument.uri, client_id, result.items, true)
 end
 
 --- Clear push diagnostics and diagnostic cache.

test/functional/plugin/lsp/diagnostic_spec.lua

@@ -470,6 +470,7 @@ describe('vim.lsp.diagnostic', function()
     end)
 
     it('requests with the `previousResultId`', function()
+      -- Full reports
       eq(
         'dummy_server',
         exec_lua(function()
@@ -497,6 +498,32 @@ describe('vim.lsp.diagnostic', function()
           return _G.params.previousResultId
         end)
       )
+
+      -- Unchanged reports
+      eq(
+        'squidward',
+        exec_lua(function()
+          vim.lsp.diagnostic.on_diagnostic(nil, {
+            kind = 'unchanged',
+            resultId = 'squidward',
+          }, {
+            method = vim.lsp.protocol.Methods.textDocument_diagnostic,
+            params = {
+              textDocument = { uri = fake_uri },
+            },
+            client_id = client_id,
+            bufnr = diagnostic_bufnr,
+          })
+          vim.api.nvim_exec_autocmds('LspNotify', {
+            buffer = diagnostic_bufnr,
+            data = {
+              method = vim.lsp.protocol.Methods.textDocument_didChange,
+              client_id = client_id,
+            },
+          })
+          return _G.params.previousResultId
+        end)
+      )
     end)
   end)
 end)