[build][clang] Ignore -Wnan-infinity-disabled in third_party code

Upstream LLVM added a stricter warning for using infinity w/ floating
point code. This patch disables the new warning in
third_party/rapidsjson, which is the only library that uses this
feature in //third_party.

Bug: 324137521
Change-Id: I5280c78613253bdd356d1b5790b5f5ca22d17e0b
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/rapidjson/+/986784
Reviewed-by: Corey Tabaka <eieio@google.com>
Commit-Queue: Paul Kirth <paulkirth@google.com>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 9ca5323..073eab9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -42,6 +42,9 @@
 
   cflags = [
     "-Wno-ambiguous-reversed-operator",
+    #TODO(fxb/324137521): Remove the Wno-unknown-warning-option after clang roll
+    "-Wno-unknown-warning-option",
+    "-Wno-nan-infinity-disabled",
     "-Wno-option-ignored",
     "-fPIC",
   ]