C++17 compatibility: make WITH_TIDY=1 happy again.

Bug: http://b/111067277
Test: builds
Change-Id: I8b69ea3815e14bb6eb27f40c0dd01a85b340a355
diff --git a/compiler/optimizing/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index e7212cd..781f272 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -2425,7 +2425,7 @@
     case DataType::Type::kUint64:
     case DataType::Type::kVoid:
       LOG(FATAL) << "Unexpected parameter type " << type;
-      break;
+      UNREACHABLE();
   }
   return Location::NoLocation();
 }