Make FunctionCallAnnotation::kind a SetOnce

libsolidity/ast/ASTAnnotations.h

@@ -27,6 +27,8 @@
 #include <libsolidity/ast/ASTEnums.h>
 #include <libsolidity/ast/ExperimentalFeatures.h>
 
+#include <libsolutil/SetOnce.h>
+
 #include <map>
 #include <memory>
 #include <optional>
@@ -285,7 +287,6 @@ struct BinaryOperationAnnotation: ExpressionAnnotation
 
 enum class FunctionCallKind
 {
-	Unset,
 	FunctionCall,
 	TypeConversion,
 	StructConstructorCall
@@ -293,7 +294,7 @@ enum class FunctionCallKind
 
 struct FunctionCallAnnotation: ExpressionAnnotation
 {
-	FunctionCallKind kind = FunctionCallKind::Unset;
+	util::SetOnce<FunctionCallKind> kind;
 	/// If true, this is the external call of a try statement.
 	bool tryCall = false;
 };