Enable -Wimplicit-fallthrough.

Falling through switch cases on a clang build must now annotate the fallthrough
with the FALLTHROUGH_INTENDED macro.
Bug: 17731372

Change-Id: I836451cd5f96b01d1ababdbf9eef677fe8fa8324
diff --git a/runtime/check_jni.cc b/runtime/check_jni.cc
index bfe44a2..fec1824 100644
--- a/runtime/check_jni.cc
+++ b/runtime/check_jni.cc
@@ -1128,7 +1128,7 @@
           *errorKind = "continuation";
           return utf8;
         }
-        // Fall through to take care of the final byte.
+        FALLTHROUGH_INTENDED;  // Fall-through to take care of the final byte.
       case 0x0c:
       case 0x0d:
         // Bit pattern 110x, so there is one additional byte.