*: 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/eval/typval.c

@@ -1869,7 +1869,7 @@ void tv_free(typval_T *tv)
       }
       case VAR_FUNC: {
         func_unref(tv->vval.v_string);
-        // FALLTHROUGH
+        FALLTHROUGH;
       }
       case VAR_STRING: {
         xfree(tv->vval.v_string);

src/nvim/macros.h

@@ -153,4 +153,22 @@
 #define STR_(x) #x
 #define STR(x) STR_(x)
 
+#ifndef __has_attribute
+# define NVIM_HAS_ATTRIBUTE(x) 0
+#elif defined(__clang__) && __clang__ == 1 \
+    && (__clang_major__ < 3 || (__clang_major__ == 3 && __clang_minor__ <= 5))
+// Starting in Clang 3.6, __has_attribute was fixed to only report true for
+// GNU-style attributes.  Prior to that, it reported true if _any_ backend
+// supported the attribute.
+# define NVIM_HAS_ATTRIBUTE(x) 0
+#else
+# define NVIM_HAS_ATTRIBUTE __has_attribute
+#endif
+
+#if NVIM_HAS_ATTRIBUTE(fallthrough)
+# define FALLTHROUGH __attribute__((fallthrough))
+#else
+# define FALLTHROUGH
+#endif
+
 #endif  // NVIM_MACROS_H

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;