Try fixing modules issue

Author: ldionne

libcxx/utils/libcxx/test/modules.py

@@ -92,6 +92,9 @@
 # same definition.
 ExtraHeader["functional"] = "v1/__compare/compare_three_way.h$"
 
+# <flat_set> reuses some functionality defined inside <flat_map>
+ExtraHeader["flat_set"] = "v1/__flat_map/sorted_unique.h$"
+
 # Some C compatibility headers define std::size_t, which is in <__cstddef/size_t.h>
 for header in ("cstdio", "cstdlib", "cstring", "ctime", "cuchar", "cwchar"):
     ExtraHeader[header] = "v1/__cstddef/size_t.h$"