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/native/java_lang_reflect_Field.cc b/runtime/native/java_lang_reflect_Field.cc
index d166be0..7f5a611 100644
--- a/runtime/native/java_lang_reflect_Field.cc
+++ b/runtime/native/java_lang_reflect_Field.cc
@@ -268,6 +268,7 @@
       break;
     }
     // Else fall through to report an error.
+    FALLTHROUGH_INTENDED;
   case Primitive::kPrimVoid:
     // Never okay.
     ThrowIllegalArgumentException(nullptr, StringPrintf("Not a primitive field: %s",