This adds a new helper that can be called from application code to
ensure that no mutexes are held on specific code paths. This is useful
for multiple scenarios, including ensuring no locks are held:
- at thread exit
- in peformance-critical code
- when a coroutine is suspended (can cause deadlocks)
See this discourse thread for more discussion:
https://discourse.llvm.org/t/add-threadsanitizer-check-to-prevent-coroutine-suspending-while-holding-a-lock-potential-deadlock/74051
This resubmits and fixes#69372 (was reverted because of build
breakage).
This also includes the followup change #71471 (to fix a land race).
1146d96096