Some refactoring and cleanup (#1806)

* Use clang-tidy `misc-include-cleaner` for IWYU `#include` cleanup

* Use `std::optional<size_t>` instead of `ssize_t`

* Rename some functions in linkdefs.hpp

* Fix header order

src/link/output.cpp

@@ -6,9 +6,12 @@
 #include <deque>
 #include <errno.h>
 #include <inttypes.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <tuple>
+#include <variant>
 #include <vector>
 
 #include "diagnostics.hpp"