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/disassembler/disassembler_x86.cc b/disassembler/disassembler_x86.cc
index 195c45f..63a74c7 100644
--- a/disassembler/disassembler_x86.cc
+++ b/disassembler/disassembler_x86.cc
@@ -412,7 +412,7 @@
         break;
       case 0x2E:
         opcode << "u";
-        // FALLTHROUGH
+        FALLTHROUGH_INTENDED;
       case 0x2F:
         if (prefix[2] == 0x66) {
           opcode << "comisd";