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/compiler/dex/quick/mir_to_lir.cc b/compiler/dex/quick/mir_to_lir.cc
index 0ac1299..4399981 100644
--- a/compiler/dex/quick/mir_to_lir.cc
+++ b/compiler/dex/quick/mir_to_lir.cc
@@ -482,7 +482,7 @@
 
     case Instruction::RETURN_OBJECT:
       DCHECK(rl_src[0].ref);
-      // Intentional fallthrough.
+      FALLTHROUGH_INTENDED;
     case Instruction::RETURN:
       if (!kLeafOptimization || !mir_graph_->MethodIsLeaf()) {
         GenSuspendTest(opt_flags);
@@ -1031,8 +1031,7 @@
         GenArithImmOpLong(opcode, rl_dest, rl_src[0], rl_src[1]);
         break;
       }
-      // Note: intentional fallthrough.
-
+      FALLTHROUGH_INTENDED;
     case Instruction::MUL_LONG:
     case Instruction::DIV_LONG:
     case Instruction::REM_LONG: