Implement -Wswitch-fallthrough: libstdc++

[Marek Polacek]

2016-07-11  Marek Polacek  <polacek@redhat.com>

	PR c/7652
	* libsupc++/hash_bytes.cc: Use __builtin_fallthrough.

diff --git gcc/libstdc++-v3/libsupc++/hash_bytes.cc gcc/libstdc++-v3/libsupc++/hash_bytes.cc
index 2e5bbfa..818331f 100644
--- gcc/libstdc++-v3/libsupc++/hash_bytes.cc
+++ gcc/libstdc++-v3/libsupc++/hash_bytes.cc
@@ -95,8 +95,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
       case 3:
 	hash ^= static_cast<unsigned char>(buf[2]) << 16;
+	__builtin_fallthrough ();
       case 2:
 	hash ^= static_cast<unsigned char>(buf[1]) << 8;
+	__builtin_fallthrough ();
       case 1:
 	hash ^= static_cast<unsigned char>(buf[0]);
 	hash *= m;