use decimal constants to avoid casting error with visual c /W4

"unary minus operator applied to unsigned type, result still unsigned"

src/google/protobuf/compiler/cpp/cpp_helpers.cc

@@ -270,9 +270,9 @@ string Int64ToString(int64 number) {
   // gcc rejects the decimal form of kint64min
   if (number == kint64min) {
     // Make sure we are in a 2's complement system.
-    GOOGLE_COMPILE_ASSERT(kint64min == GOOGLE_LONGLONG(-0x8000000000000000),
+    GOOGLE_COMPILE_ASSERT(kint64min == GOOGLE_LONGLONG(~0x7fffffffffffffff),
                    kint64min_value_error);
-    return "GOOGLE_LONGLONG(-0x8000000000000000)";
+    return "GOOGLE_LONGLONG(~0x7fffffffffffffff)";
   }
   return "GOOGLE_LONGLONG(" + SimpleItoa(number) + ")";
 }

src/google/protobuf/compiler/cpp/cpp_unittest.cc

@@ -159,7 +159,7 @@ TEST(GeneratedMessageTest, Int32StringConversion) {
 TEST(GeneratedMessageTest, Int64StringConversion) {
   EXPECT_EQ("GOOGLE_LONGLONG(971)", Int64ToString(971));
   EXPECT_EQ("GOOGLE_LONGLONG(-2147483648)", Int64ToString(kint32min));
-  EXPECT_EQ("GOOGLE_LONGLONG(-0x8000000000000000)", Int64ToString(kint64min));
+  EXPECT_EQ("GOOGLE_LONGLONG(~0x7fffffffffffffff)", Int64ToString(kint64min));
   EXPECT_EQ("GOOGLE_LONGLONG(9223372036854775807)", Int64ToString(kint64max));
 }
 
@@ -196,8 +196,8 @@ TEST(GeneratedMessageTest, Trigraph) {
 TEST(GeneratedMessageTest, ExtremeSmallIntegerDefault) {
   const unittest::TestExtremeDefaultValues& extreme_default =
       unittest::TestExtremeDefaultValues::default_instance();
-  EXPECT_EQ(-0x80000000, kint32min);
-  EXPECT_EQ(GOOGLE_LONGLONG(-0x8000000000000000), kint64min);
+  EXPECT_EQ(~0x7fffffff, kint32min);
+  EXPECT_EQ(GOOGLE_LONGLONG(~0x7fffffffffffffff), kint64min);
   EXPECT_EQ(kint32min, extreme_default.really_small_int32());
   EXPECT_EQ(kint64min, extreme_default.really_small_int64());
 }