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/dex_file_verifier.cc b/runtime/dex_file_verifier.cc
index 0597253..a3f3de8 100644
--- a/runtime/dex_file_verifier.cc
+++ b/runtime/dex_file_verifier.cc
@@ -142,7 +142,7 @@
         ErrorStringPrintf("Invalid use of void");
         return false;
       }
-      // Intentional fallthrough.
+      FALLTHROUGH_INTENDED;
     case 'B':
     case 'C':
     case 'D':