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/fix/main.cpp

@@ -1,7 +1,6 @@
 // SPDX-License-Identifier: MIT
 
 #include <sys/stat.h>
-#include <sys/types.h>
 
 #include <errno.h>
 #include <limits.h>

src/fix/mbc.cpp

@@ -2,6 +2,8 @@
 
 #include "fix/mbc.hpp"
 
+#include <stdint.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <unordered_map>
 #include <utility>

src/fix/warning.cpp

@@ -2,6 +2,13 @@
 
 #include "fix/warning.hpp"
 
+#include <inttypes.h>
+#include <stdarg.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "diagnostics.hpp"
 #include "style.hpp"
 
 // clang-format off: nested initializers