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/warning.cpp

@@ -4,7 +4,11 @@
 
 #include <inttypes.h>
 #include <stdarg.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
 
+#include "diagnostics.hpp"
 #include "style.hpp"
 
 #include "link/fstack.hpp"