Clean up language server's output handling (#4855)

Handles verbose logging and printing errors as diagnostics to stderr
when there's no way to communicate them back on a request.

toolchain/language_server/language_server.cpp

@@ -15,12 +15,39 @@
 
 namespace Carbon::LanguageServer {
 
+// An adapter for clangd's logging that sends most messages to `error_stream`.
+// Verbose logging is only printed if `vlog_stream` is provided, and will be
+// sent there.
+class Logger : public clang::clangd::Logger {
+ public:
+  explicit Logger(llvm::raw_ostream* error_stream,
+                  llvm::raw_ostream* vlog_stream)
+      : error_logger_(*error_stream, clang::clangd::Logger::Info),
+        vlog_logger_(vlog_stream
+                         ? std::make_unique<clang::clangd::StreamLogger>(
+                               *vlog_stream, clang::clangd::Logger::Verbose)
+                         : nullptr) {}
+
+  auto log(Level level, const char* format,
+           const llvm::formatv_object_base& message) -> void override {
+    if (level != clang::clangd::Logger::Verbose) {
+      error_logger_.log(level, format, message);
+    } else if (vlog_logger_) {
+      vlog_logger_->log(level, format, message);
+    }
+  }
+
+ private:
+  clang::clangd::StreamLogger error_logger_;
+  std::unique_ptr<clang::clangd::StreamLogger> vlog_logger_;
+};
+
 auto Run(FILE* input_stream, llvm::raw_ostream& output_stream,
-         llvm::raw_ostream& error_stream, DiagnosticConsumer& consumer)
-    -> bool {
-  // TODO: Consider implementing a custom logger that splits vlog to
-  // vlog_stream when provided. For now, this disables verbose logging.
-  clang::clangd::StreamLogger logger(error_stream, clang::clangd::Logger::Info);
+         llvm::raw_ostream& error_stream, llvm::raw_ostream* vlog_stream,
+         DiagnosticConsumer& consumer) -> bool {
+  // The language server internally uses diagnostics for logging issues, but the
+  // clangd parts have their own logging system. We intercept that here.
+  Logger logger(&error_stream, vlog_stream);
   clang::clangd::LoggingSession logging_session(logger);
 
   // Set up the connection.
@@ -28,7 +55,7 @@ auto Run(FILE* input_stream, llvm::raw_ostream& output_stream,
       clang::clangd::newJSONTransport(input_stream, output_stream,
                                       /*InMirror=*/nullptr,
                                       /*Pretty=*/true));
-  Context context;
+  Context context(&consumer);
   // TODO: Use error_stream in IncomingMessages to report dropped errors.
   IncomingMessages incoming(transport.get(), &context);
   OutgoingMessages outgoing(transport.get());