building with gcc trunk shows:

/home/markus/llvm/projects/compiler-rt/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc: In function ‘int main(int, char**)’:
/home/markus/llvm/projects/compiler-rt/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc:284:30: error: ‘function’ is not a member of ‘std’
   std::map<std::string, std::function<void(std::istream &)>> TopLevelRecordMap;
                              ^~~~~~~~
/home/markus/llvm/projects/compiler-rt/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc:284:30: note: suggested alternative: ‘is_function’
   std::map<std::string, std::function<void(std::istream &)>> TopLevelRecordMap;
                              ^~~~~~~~
                              is_function
/home/markus/llvm/projects/compiler-rt/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc:284:30: error: ‘function’ is not a member of ‘std’
/home/markus/llvm/projects/compiler-rt/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc:284:30: note: suggested alternative: ‘is_function’
   std::map<std::string, std::function<void(std::istream &)>> TopLevelRecordMap;
                              ^~~~~~~~
                              is_function
/home/markus/llvm/projects/compiler-rt/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc:284:59: error: template argument 2 is invalid
   std::map<std::string, std::function<void(std::istream &)>> TopLevelRecordMap;
                                                           ^~
/home/markus/llvm/projects/compiler-rt/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc:284:59: error: template argument 4 is invalid
...

diff --git a/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc b/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc
index a4d05f200de5..6e209809e346 100644
--- a/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc
+++ b/lib/xray/tests/unit/xray_fdr_log_printer_tool.cc
@@ -17,6 +17,7 @@
 #include <algorithm>
 #include <array>
 #include <cstdlib>
+#include <functional>
 #include <iostream>
 #include <map>
 #include <sstream>

fixes the issue.