[clang-tidy] use nodiscard

Introduced in C++17. It replaces gcc's warn_unused_result.

Found with modernize-use-nodiscard.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/storage/plugins/CurlStorage.cxx

@@ -169,7 +169,7 @@ struct DavResponse {
 		std::chrono::system_clock::time_point::min();
 	uint64_t length = 0;
 
-	bool Check() const {
+	[[nodiscard]] bool Check() const {
 		return !href.empty();
 	}
 };

src/storage/plugins/NfsStorage.cxx

@@ -129,7 +129,7 @@ public:
 	}
 
 private:
-	EventLoop &GetEventLoop() const noexcept {
+	[[nodiscard]] EventLoop &GetEventLoop() const noexcept {
 		return defer_connect.GetEventLoop();
 	}
 
@@ -276,7 +276,7 @@ public:
 		:BlockingNfsOperation(_connection), path(_path),
 		 follow(_follow) {}
 
-	const StorageFileInfo &GetInfo() const {
+	[[nodiscard]] const StorageFileInfo &GetInfo() const {
 		return info;
 	}