fix(diagnostic): preserve fields from LSP diagnostics via user_data (#15735)

* preserve fields from LSP diagnostics via adding a user_data table to the diagnostic, which can hold arbitrary data in addition to the lsp diagnostic information.

test/functional/plugin/lsp/diagnostic_spec.lua

@@ -428,6 +428,32 @@ describe('vim.lsp.diagnostic', function()
         end)
       end)
     end)
+
+    it('maintains LSP information when translating diagnostics', function()
+      local result = exec_lua [[
+        local diagnostics = {
+          make_error("Error 1", 1, 1, 1, 5),
+        }
+
+        diagnostics[1].code = 42
+        diagnostics[1].tags = {"foo", "bar"}
+        diagnostics[1].data = "Hello world"
+
+        vim.lsp.diagnostic.on_publish_diagnostics(nil, {
+          uri = fake_uri,
+          diagnostics = diagnostics,
+        }, {client_id=1})
+
+        return {
+          vim.diagnostic.get(diagnostic_bufnr, {lnum=1})[1],
+          vim.lsp.diagnostic.get_line_diagnostics(diagnostic_bufnr, 1)[1],
+        }
+      ]]
+      eq({code = 42, tags = {"foo", "bar"}, data = "Hello world"}, result[1].user_data.lsp)
+      eq(42, result[2].code)
+      eq({"foo", "bar"}, result[2].tags)
+      eq("Hello world", result[2].data)
+    end)
   end)
 
   describe("vim.lsp.diagnostic.get_line_diagnostics", function()