Fix CMake build and lint warnings in field_value.cc (#677)

Fixes these cpplint warnings:

Firestore/core/src/firebase/firestore/model/field_value.cc:162:  Semicolon defining empty statement. Use {} instead.  [whitespace/semicolon] [5]
Firestore/core/src/firebase/firestore/model/field_value.cc:170:  Semicolon defining empty statement. Use {} instead.  [whitespace/semicolon] [5]
Firestore/core/src/firebase/firestore/model/field_value.cc:126:  Add #include <utility> for swap  [build/include_what_you_use] [4]

1 files changed, 3 insertions, 2 deletions

diff --git a/Firestore/core/src/firebase/firestore/model/field_value.cc b/Firestore/core/src/firebase/firestore/model/field_value.cc
index 5a19549..45886bf 100644
--- a/Firestore/core/src/firebase/firestore/model/field_value.cc
+++ b/Firestore/core/src/firebase/firestore/model/field_value.cc
@@ -18,6 +18,7 @@
 
 #include <algorithm>
 #include <memory>
+#include <utility>
 #include <vector>
 
 #include "Firestore/core/src/firebase/firestore/util/firebase_assert.h"
@@ -159,7 +160,7 @@ void FieldValue::SwitchTo(const Type type) {
     case Type::Array:
       array_value_.~vector();
       break;
-    default:;  // The other types where there is nothing to worry about.
+    default: {}  // The other types where there is nothing to worry about.
   }
   tag_ = type;
   // Must call constructor explicitly for any non-POD type to initialize.
@@ -167,7 +168,7 @@ void FieldValue::SwitchTo(const Type type) {
     case Type::Array:
       new (&array_value_) std::vector<const FieldValue>();
       break;
-    default:;  // The other types where there is nothing to worry about.
+    default: {}  // The other types where there is nothing to worry about.
   }
 }