*: Use __attribute__((fallthrough)) where comments aren't supported

Although GCC now detects possibly unintentional fall through, there
rules around which the comments are detected are rather strict.  In
cases where a comment isn't detected, upstream [recommends] using their
fallthrough attribute.

[recommends]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77817#c11

src/nvim/shada.c

@@ -2047,7 +2047,7 @@ static inline ShaDaWriteResult shada_read_when_writing(
       }
       case kSDReadStatusNotShaDa: {
         ret = kSDWriteReadNotShada;
-        // fallthrough
+        FALLTHROUGH;
       }
       case kSDReadStatusReadError: {
         return ret;