[gn build] Port 1d131ff

Author: aeubanks

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -1568,6 +1568,7 @@ if (current_toolchain == default_toolchain) {
       "__utility/cmp.h",
       "__utility/convert_to_integral.h",
       "__utility/declval.h",
+      "__utility/default_three_way_comparator.h",
       "__utility/element_count.h",
       "__utility/empty.h",
       "__utility/exception_guard.h",
@@ -1578,6 +1579,7 @@ if (current_toolchain == default_toolchain) {
       "__utility/integer_sequence.h",
       "__utility/is_pointer_in_range.h",
       "__utility/is_valid_range.h",
+      "__utility/lazy_synth_three_way_comparator.h",
       "__utility/move.h",
       "__utility/no_destroy.h",
       "__utility/pair.h",