firestore: prepare for llvm 21 (#13932)

Ref:
#13915 (comment)

---------

Signed-off-by: David Korczynski <[email protected]>

Author: DavidKorczynski

projects/firestore/build.sh

@@ -19,8 +19,8 @@ cd $WORK
 
 # Disable UBSan vptr since Firestore depends on other libraries that are built
 # with -fno-rtti.
-export CFLAGS="$CFLAGS -fno-sanitize=vptr"
-export CXXFLAGS="$CXXFLAGS -fno-sanitize=vptr"
+export CFLAGS="$CFLAGS -Wno-error=unknown-warning-option -fno-sanitize=vptr -Wno-error=missing-template-arg-list-after-template-kw -Wno-error=deprecated-builtins -Wno-error=deprecated-literal-operator"
+export CXXFLAGS="$CXXFLAGS -Wno-error=unknown-warning-option -fno-sanitize=vptr -Wno-error=missing-template-arg-list-after-template-kw -Wno-error=deprecated-builtins -Wno-error=deprecated-literal-operator"
 
 # Build the project using cmake with FUZZING option enabled to link to OSS Fuzz
 # fuzzing library defined in ${LIB_FUZZING_ENGINE}.