[rapidjson] Disable ambiguous operator warning for C++20.

The standard is a bit overzealous in what it disallows now that
operator<=> is a thing; rapidjson's usage is perfectly unambiguous but
still disallowed by the standard, but luckily there is a flag to allow
it anyway.

Test: Build works on C++20
Change-Id: Ie4bd73ac044f0d09d4474545cea3cdfed16e8517
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/rapidjson/+/632402
Reviewed-by: Roland McGrath <mcgrathr@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 92b7a01..4de322e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -40,6 +40,8 @@
     "RAPIDJSON_HAS_CXX11_TYPETRAITS",
     "RAPIDJSON_HAS_CXX11_NOEXCEPT",
   ]
+
+  cflags = [ "-Wno-ambiguous-reversed-operator" ]
 }
 
 source_set("rapidjson") {