second part of #11755

[dsyme]

#11755 contained two fixes - the second noticed when I ramped up the testing for debug mode code.

However we want to take the minimal fix for the regression, and the second bug fix is not complete

So this is tracking the second bug fix

[dsyme]

Random unrelated test failure

2021-08-12T11:45:58.1366610Z [xUnit.net 00:01:37.43] FSharp.Core.UnitTests.Control.AsyncModule.AwaitWaitHandle.Timeout [FAIL]
2021-08-12T11:45:58.7229320Z Failed FSharp.Core.UnitTests.Control.AsyncModule.AwaitWaitHandle.Timeout [1 s]
2021-08-12T11:45:58.7233710Z Error Message:
2021-08-12T11:45:58.7234660Z Expected faster timeout than 1505 ms
2021-08-12T11:45:58.7235730Z Expected: True
2021-08-12T11:45:58.7236740Z Actual: False
2021-08-12T11:45:58.7237720Z Stack Trace:
2021-08-12T11:45:58.7238960Z at FSharp.Core.UnitTests.Control.AsyncModule.AwaitWaitHandle.Timeout() in /Users/runner/work/1/s/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/AsyncModule.fs:line 232