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/optimizing/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index 393eb1a..c15cca6 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -1158,9 +1158,10 @@
     case Primitive::kPrimFloat:
     case Primitive::kPrimDouble:
       LOG(FATAL) << "Unimplemented register type " << field_type;
-
+      UNREACHABLE();
     case Primitive::kPrimVoid:
       LOG(FATAL) << "Unreachable type " << field_type;
+      UNREACHABLE();
   }
 }
 
@@ -1212,9 +1213,10 @@
     case Primitive::kPrimFloat:
     case Primitive::kPrimDouble:
       LOG(FATAL) << "Unimplemented register type " << instruction->GetType();
-
+      UNREACHABLE();
     case Primitive::kPrimVoid:
       LOG(FATAL) << "Unreachable type " << instruction->GetType();
+      UNREACHABLE();
   }
 }
 
@@ -1339,9 +1341,10 @@
     case Primitive::kPrimFloat:
     case Primitive::kPrimDouble:
       LOG(FATAL) << "Unimplemented register type " << instruction->GetType();
-
+      UNREACHABLE();
     case Primitive::kPrimVoid:
       LOG(FATAL) << "Unreachable type " << instruction->GetType();
+      UNREACHABLE();
   }
 }
 
@@ -1465,9 +1468,10 @@
     case Primitive::kPrimFloat:
     case Primitive::kPrimDouble:
       LOG(FATAL) << "Unimplemented register type " << instruction->GetType();
-
+      UNREACHABLE();
     case Primitive::kPrimVoid:
       LOG(FATAL) << "Unreachable type " << instruction->GetType();
+      UNREACHABLE();
   }
 }