Replace HARD_ASSERT with SIMPLE_HARD_ASSERT to fix absl linker breakage.

[dconeybe]

Fix the following build error in the downstream Unity SDK builds for iOS:

Undefined symbols for architecture arm64:
  "firebase::firestore::util::internal::StringFormatPieces(char const*, std::initializer_list<absl::string_view>)", referenced from:
      std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > firebase::firestore::util::StringFormat<char const*>(char const*, char const* const&) in libFirebaseCppFirestore.a(firestore_main.o)
      firebase::firestore::TransactionInternal::ValidateReference(firebase::firestore::DocumentReference const&) in libFirebaseCppFirestore.a(transaction_main.o)
      void firebase::firestore::util::ThrowInvalidArgument<>(char const*) in libFirebaseCppFirestore.a(transaction_main.o)
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[github-actions]

❌  Integration test FAILED

Requested by @dconeybe on commit 48103b1
Last updated: Tue Aug 24 14:14 PDT 2021
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FAILURE] [Android] [windows] [android_target] (1 failed tests)   ServerTimestampTest.TestServerTimestampsUsesPreviousValueFromLocalMutation

Add flaky tests to go/fpl-cpp-flake-tracker