Make _Differentiable protocol compiler-known.

The `_Differentiable` protocol is compiler-known and will be used during
type-checking, SILGen, and the SIL differentiation transform.

Author: dan-zheng

include/swift/AST/KnownProtocols.def

@@ -83,6 +83,8 @@ PROTOCOL_(DestructorSafeContainer)
 
 PROTOCOL(StringInterpolationProtocol)
 
+PROTOCOL_(Differentiable)
+
 EXPRESSIBLE_BY_LITERAL_PROTOCOL(ExpressibleByArrayLiteral, "Array", false)
 EXPRESSIBLE_BY_LITERAL_PROTOCOL(ExpressibleByBooleanLiteral, "BooleanLiteralType", true)
 EXPRESSIBLE_BY_LITERAL_PROTOCOL(ExpressibleByDictionaryLiteral, "Dictionary", false)

lib/IRGen/GenMeta.cpp

@@ -4217,6 +4217,7 @@ SpecialProtocol irgen::getSpecialProtocolID(ProtocolDecl *P) {
   case KnownProtocolKind::Encodable:
   case KnownProtocolKind::Decodable:
   case KnownProtocolKind::StringInterpolationProtocol:
+  case KnownProtocolKind::Differentiable:
     return SpecialProtocol::None;
   }