refactor: fix clang-tidy warnings

Enable and fix bugprone-misplaced-widening-cast warning.

Fix some modernize-macro-to-enum and readability-else-after-return
warnings, but don't enable them. While the warnings can be useful, they
are in general too noisy to enable.

src/nvim/message.c

@@ -59,8 +59,10 @@ struct msgchunk_S {
 };
 
 // Magic chars used in confirm dialog strings
-#define DLG_BUTTON_SEP  '\n'
-#define DLG_HOTKEY_CHAR '&'
+enum {
+  DLG_BUTTON_SEP = '\n',
+  DLG_HOTKEY_CHAR = '&',
+};
 
 static int confirm_msg_used = false;            // displaying confirm_msg
 #ifdef INCLUDE_GENERATED_DECLARATIONS