util: Add support for clang::fallthrough.

Looks like the __attribute__ version doesn't work for C++ in the
Android build. Only found now because we don't enable
-Wimplicit-fallthrough by default project wide for C++. Only
ACO enables it.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13164>
(cherry picked from commit aad80e47d8)

.pick_status.json

@@ -202,7 +202,7 @@
         "description": "util: Add support for clang::fallthrough.",
         "nominated": false,
         "nomination_type": null,
-        "resolution": 4,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null
     },

src/util/compiler.h

@@ -76,9 +76,20 @@
 #  define __has_attribute(x) 0
 #endif
 
+#if defined(__has_cpp_attribute) && defined(__clang__)
+/* We do not do the same trick as __has_attribute because parsing
+ * clang::fallthrough in the preprocessor fails in GCC. */
+#  define HAS_CLANG_FALLTHROUGH  __has_cpp_attribute(clang::fallthrough)
+#else
+#  define HAS_CLANG_FALLTHROUGH 0
+#endif
+
 #if __cplusplus >= 201703L || __STDC_VERSION__ > 201710L
 /* Standard C++17/C23 attribute */
 #define FALLTHROUGH [[fallthrough]]
+#elif HAS_CLANG_FALLTHROUGH
+/* Clang++ specific */
+#define FALLTHROUGH [[clang::fallthrough]]
 #elif __has_attribute(fallthrough)
 /* Non-standard but supported by at least gcc and clang */
 #define FALLTHROUGH __attribute__((fallthrough))