Fix enum writing.

Enums use varint, so if the value is negative it should end up being longer.

This was caught my new conformance test cases.

conformance/failure_list_objc.txt

@@ -1,16 +1,2 @@
 # JSON input or output tests are skipped (in conformance_objc.m) as mobile
 # platforms don't support JSON wire format to avoid code bloat.
-Recommended.Proto2.ProtobufInput.ValidDataRepeated.ENUM.PackedInput.DefaultOutput.ProtobufOutput
-Recommended.Proto2.ProtobufInput.ValidDataRepeated.ENUM.PackedInput.PackedOutput.ProtobufOutput
-Recommended.Proto2.ProtobufInput.ValidDataRepeated.ENUM.PackedInput.UnpackedOutput.ProtobufOutput
-Recommended.Proto2.ProtobufInput.ValidDataRepeated.ENUM.UnpackedInput.DefaultOutput.ProtobufOutput
-Recommended.Proto2.ProtobufInput.ValidDataRepeated.ENUM.UnpackedInput.PackedOutput.ProtobufOutput
-Recommended.Proto2.ProtobufInput.ValidDataRepeated.ENUM.UnpackedInput.UnpackedOutput.ProtobufOutput
-Recommended.Proto2.ProtobufInput.ValidDataScalarBinary.ENUM[3].ProtobufOutput
-Recommended.Proto3.ProtobufInput.ValidDataRepeated.ENUM.PackedInput.DefaultOutput.ProtobufOutput
-Recommended.Proto3.ProtobufInput.ValidDataRepeated.ENUM.PackedInput.PackedOutput.ProtobufOutput
-Recommended.Proto3.ProtobufInput.ValidDataRepeated.ENUM.PackedInput.UnpackedOutput.ProtobufOutput
-Recommended.Proto3.ProtobufInput.ValidDataRepeated.ENUM.UnpackedInput.DefaultOutput.ProtobufOutput
-Recommended.Proto3.ProtobufInput.ValidDataRepeated.ENUM.UnpackedInput.PackedOutput.ProtobufOutput
-Recommended.Proto3.ProtobufInput.ValidDataRepeated.ENUM.UnpackedInput.UnpackedOutput.ProtobufOutput
-Recommended.Proto3.ProtobufInput.ValidDataScalarBinary.ENUM[3].ProtobufOutput

objectivec/GPBCodedOutputStream.m

@@ -374,12 +374,12 @@ static void GPBWriteRawLittleEndian64(GPBOutputBufferState *state,
 }
 
 - (void)writeEnumNoTag:(int32_t)value {
-  GPBWriteRawVarint32(&state_, value);
+  GPBWriteInt32NoTag(&state_, value);
 }
 
 - (void)writeEnum:(int32_t)fieldNumber value:(int32_t)value {
   GPBWriteTagWithFormat(&state_, fieldNumber, GPBWireFormatVarint);
-  GPBWriteRawVarint32(&state_, value);
+  GPBWriteInt32NoTag(&state_, value);
 }
 
 - (void)writeSFixed32NoTag:(int32_t)value {
@@ -1053,7 +1053,7 @@ size_t GPBComputeUInt32SizeNoTag(int32_t value) {
 }
 
 size_t GPBComputeEnumSizeNoTag(int32_t value) {
-  return GPBComputeRawVarint32Size(value);
+  return GPBComputeInt32SizeNoTag(value);
 }
 
 size_t GPBComputeSFixed32SizeNoTag(int32_t value) {

objectivec/Tests/GPBCodedOuputStreamTests.m

@@ -237,7 +237,15 @@
 }
 
 - (void)testWriteVarint5 {
-  // 2961488830
+  // The sign/nosign distinction is done here because normally varints are
+  // around 64bit values, but for some cases a 32bit value is forced with
+  // with the sign bit (tags, uint32, etc.)
+
+  // 1887747006 (no sign bit)
+  [self assertWriteVarint:bytes(0xbe, 0xf7, 0x92, 0x84, 0x07)
+                    value:(0x3e << 0) | (0x77 << 7) | (0x12 << 14) |
+                          (0x04 << 21) | (0x07LL << 28)];
+  // 2961488830 (sign bit)
   [self assertWriteVarint:bytes(0xbe, 0xf7, 0x92, 0x84, 0x0b)
                     value:(0x3e << 0) | (0x77 << 7) | (0x12 << 14) |
                           (0x04 << 21) | (0x0bLL << 28)];