438316f98d
The behavior of acquire+release annotation handling has changed in https://reviews.llvm.org/D49355 which breaks the build with the new Clang. However, as has been pointed out, the acquire+release isn't the right way to prevent double locking as the annotations negate each other; the correct way is to use excludes or negative requires. Using excludes annotations also requires using std::lock_guard instead of std::unique_lock because the latter doesn't have the thread annotations due to deferred locking which is not needed in Flutter and so std::lock_guard is a sufficient alternative.