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/arm/utility_arm.cc b/compiler/dex/quick/arm/utility_arm.cc
index bba1a8c..e833c9a 100644
--- a/compiler/dex/quick/arm/utility_arm.cc
+++ b/compiler/dex/quick/arm/utility_arm.cc
@@ -494,7 +494,7 @@
           (value <= 1020) && ((value & 0x3) == 0)) {
         return NewLIR3(kThumbAddPcRel, r_dest.GetReg(), r_src1.GetReg(), value >> 2);
       }
-      // Note: intentional fallthrough
+      FALLTHROUGH_INTENDED;
     case kOpSub:
       if (all_low_regs && ((abs_value & 0x7) == abs_value)) {
         if (op == kOpAdd)