Add -Wno-error=thread-safety-reference-return to flags

Fixes envoy build

infra/base-images/base-clang/Dockerfile

@@ -61,7 +61,7 @@ ENV CCC "clang++"
 # See https://releases.llvm.org/16.0.0/tools/clang/docs/ReleaseNotes.html#potentially-breaking-changes
 # Same for deprecated-declarations, int-conversion,
 # incompatible-function-pointer-types, enum-constexpr-conversion,
-# vla-cxx-extension, format-truncation
+# vla-cxx-extension, format-truncation, thread-safety-reference-return
 
 ENV CFLAGS -O1 \
   -fno-omit-frame-pointer \
@@ -74,6 +74,7 @@ ENV CFLAGS -O1 \
   -Wno-error=implicit-int \
   -Wno-error=vla-cxx-extension \
   -Wno-error=format-truncation \
+  -Wno-error=thread-safety-reference-return \
   -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
 ENV CXXFLAGS_EXTRA "-stdlib=libc++"
 ENV CXXFLAGS "$CFLAGS $CXXFLAGS_EXTRA"