Fix MSVC truncation of constant value

Summary:
This warning:
```
thrift\lib\cpp2\protocol\compactprotocol.h(47): warning C4309: 'static_cast': truncation of constant value
thrift\lib\cpp2\protocol\compactprotocol.h(48): warning C4309: 'static_cast': truncation of constant value
```
happens when `static_cast`ing a number >= 128 into a negative 8-bit value. Fix it using an unrestricted cast.

For clarity, removing the unnecesssary cast on 0x02 just above it.

Reviewed By: yfeldblum

Differential Revision: D17789349

fbshipit-source-id: aebd3688ae9592783fea892a9a04e24b16c38f51

thrift/lib/cpp2/protocol/CompactProtocol.h

@@ -42,10 +42,10 @@ using folly::io::QueueAppender;
 namespace detail {
 namespace compact {
 
-static const int8_t COMPACT_PROTOCOL_VERSION = static_cast<int8_t>(0x02);
+static const int8_t COMPACT_PROTOCOL_VERSION = 0x02;
 static const int32_t VERSION_2 = 0x82020000;
-static const int8_t PROTOCOL_ID = static_cast<int8_t>(0x82);
-static const int8_t TYPE_MASK = static_cast<int8_t>(0xE0);
+static const int8_t PROTOCOL_ID = int8_t(0x82);
+static const int8_t TYPE_MASK = int8_t(0xE0);
 static const int32_t TYPE_SHIFT_AMOUNT = 5;
 
 // Simple stack with an inline buffer for built-in types