IRGen: LinkEntity::Kind::DifferentiabilityWitness is not a type kind

As the above comments imply, when isTypeKind() returns true, then
getType() should return a valid Type. This wasn't true with
DifferentiabilityWitness.

Author: slavapestov

include/swift/IRGen/Linking.h

@@ -450,6 +450,10 @@ class LinkEntity {
     // These are both type kinds and protocol-conformance kinds.
     // TYPE KINDS: BEGIN {{
 
+    /// A SIL differentiability witness. The pointer is a
+    /// SILDifferentiabilityWitness*.
+    DifferentiabilityWitness,
+
     /// A lazy protocol witness accessor function. The pointer is a
     /// canonical TypeBase*, and the secondary pointer is a
     /// ProtocolConformance*.
@@ -460,10 +464,6 @@ class LinkEntity {
     /// ProtocolConformance*.
     ProtocolWitnessTableLazyCacheVariable,
 
-    /// A SIL differentiability witness. The pointer is a
-    /// SILDifferentiabilityWitness*.
-    DifferentiabilityWitness,
-
     // Everything following this is a type kind.
 
     /// A value witness for a type.