Add MOZ_FALLTHROUGH macro definition for gcc 7 to suppress -Wimplicit-fallthrough warnings

The generic fallback MOZ_FALLTHROUGH definition is insufficient for GCC 7 and above, resulting in build warning spam and --enable-warnings-as-errors builds failing. The check for clang support is changed to use the __has_cpp_attribute macro, which is more robust than checking the __cplusplus version. Also, MOZ_FALLTHROUGH is now only defined in C++ code, since GCC errors out if it encounters a scoped attribute being used with __has_cpp_attribute in C code. No C code uses MOZ_FALLTHROUGH or derivatives at the moment.

mfbt/Attributes.h

@@ -256,6 +256,8 @@
 #  define MOZ_MUST_USE
 #endif
 
+#ifdef __cplusplus
+
 /**
  * MOZ_FALLTHROUGH is an annotation to suppress compiler warnings about switch
  * cases that fall through without a break or return statement. MOZ_FALLTHROUGH
@@ -282,9 +284,14 @@
  *     return 5;
  * }
  */
-#if defined(__clang__) && __cplusplus >= 201103L
-   /* clang's fallthrough annotations are only available starting in C++11. */
+#ifndef __has_cpp_attribute
+#  define __has_cpp_attribute(x) 0
+#endif
+
+#if __has_cpp_attribute(clang::fallthrough)
 #  define MOZ_FALLTHROUGH [[clang::fallthrough]]
+#elif __has_cpp_attribute(gnu::fallthrough)
+#  define MOZ_FALLTHROUGH [[gnu::fallthrough]]
 #elif defined(_MSC_VER)
    /*
     * MSVC's __fallthrough annotations are checked by /analyze (Code Analysis):
@@ -296,8 +303,6 @@
 #  define MOZ_FALLTHROUGH /* FALLTHROUGH */
 #endif
 
-#ifdef __cplusplus
-
 /*
  * The following macros are attributes that support the static analysis plugin
  * included with Mozilla, and will be implemented (when such support is enabled)