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/assemble_arm.cc b/compiler/dex/quick/arm/assemble_arm.cc
index cf34948..4e20d76 100644
--- a/compiler/dex/quick/arm/assemble_arm.cc
+++ b/compiler/dex/quick/arm/assemble_arm.cc
@@ -1265,7 +1265,7 @@
           if (lir->operands[1] != rs_r15pc.GetReg()) {
             break;
           }
-          // NOTE: intentional fallthrough.
+          FALLTHROUGH_INTENDED;
         case kFixupLoad: {
           /*
            * PC-relative loads are mostly used to load immediates