build: allow IWYU to fix includes for all .c files

Allow Include What You Use to remove unnecessary includes and only
include what is necessary. This helps with reducing compilation times
and makes it easier to visualise which dependencies are actually
required.

Work on https://github.com/neovim/neovim/issues/549, but doesn't close
it since this only works fully for .c files and not headers.

src/nvim/mbyte.c

@@ -25,36 +25,55 @@
 /// Vim scripts may contain an ":scriptencoding" command. This has an effect
 /// for some commands, like ":menutrans".
 
-#include <inttypes.h>
+#include <assert.h>
+#include <ctype.h>
+#include <errno.h>
+#include <iconv.h>
 #include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <wchar.h>
 #include <wctype.h>
 
-#include "nvim/ascii.h"
-#include "nvim/vim.h"
-#ifdef HAVE_LOCALE_H
-# include <locale.h>
-#endif
+#include "auto/config.h"
 #include "nvim/arabic.h"
+#include "nvim/ascii.h"
+#include "nvim/buffer_defs.h"
 #include "nvim/charset.h"
 #include "nvim/cursor.h"
 #include "nvim/drawscreen.h"
-#include "nvim/eval.h"
-#include "nvim/fileio.h"
-#include "nvim/func_attr.h"
+#include "nvim/eval/typval.h"
+#include "nvim/eval/typval_defs.h"
 #include "nvim/getchar.h"
+#include "nvim/gettext.h"
+#include "nvim/globals.h"
+#include "nvim/grid_defs.h"
 #include "nvim/iconv.h"
+#include "nvim/keycodes.h"
+#include "nvim/macros.h"
 #include "nvim/mark.h"
 #include "nvim/mbyte.h"
+#include "nvim/mbyte_defs.h"
 #include "nvim/memline.h"
 #include "nvim/memory.h"
 #include "nvim/message.h"
+#include "nvim/option_defs.h"
 #include "nvim/os/os.h"
-#include "nvim/path.h"
+#include "nvim/os/os_defs.h"
+#include "nvim/pos.h"
 #include "nvim/screen.h"
-#include "nvim/spell.h"
 #include "nvim/strings.h"
+#include "nvim/types.h"
+#include "nvim/vim.h"
+
+#ifdef HAVE_LOCALE_H
+# include <locale.h>
+#endif
+
+#ifdef __STDC_ISO_10646__
+# include <stdc-predef.h>
+#endif
 
 typedef struct {
   int rangeStart;
@@ -68,11 +87,12 @@ struct interval {
   long last;
 };
 
+// uncrustify:off
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "mbyte.c.generated.h"
-
 # include "unicode_tables.generated.h"
 #endif
+// uncrustify:on
 
 static char e_list_item_nr_is_not_list[]
   = N_("E1109: List item %d is not a List");