Fix issue 15917 - std.concurrency module destructor causes useless cr…

[yazd]

…eation of new MessageBox.

[dlang-bot]

Fix	Bugzilla	Description
✓	15917	std.concurrency module destructor causes useless creation of new MessageBox

[yazd]

To help in reviewing the code, what happens is that the module destructor calls TheadInfo.cleanup which in turn calls unregisterMe which, in order to do its job, requires the current thread's Tid. So it calls thisTid, but thisTid constructs a Tid lazily when first called and so it creates a new MessageBox which in turns creates a load of unnecessary stuff.
The solution in this PR is to, instead of using thisTid in unregisterMe, directly check the existence of the Tid using thisInfo.ident (which is used in thisTid).

This bug is indirectly causing this issue in vibe.d. Its fix requires this fix in Phobos.

[schveiguy]

LGTM. Auto tester seems to be having hiccups with timeouts on freebsd.

[DmitryOlshansky]

LGTM

[DmitryOlshansky]

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