[compiler-rt] [asan][win] Fix CreateThread leak (PR #126738)

[Reid Kleckner via llvm-commits]

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@@ -143,9 +143,11 @@ static thread_return_t THREAD_CALLING_CONV asan_thread_start(void *arg) {
 
   ThreadStartParams params;
   t->GetStartData(params);
+  // The ExitThread will end the current thread, causing destroy to be unable to
+  // be called.
+  t->Destroy();
----------------
rnk wrote:

I think `t` is escaped above, so this isn't correct, it needs to happen after the thread runs.

If the user thread routine calls [ExitThread](https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-exitthread) manually rather than returning, then the proper fix for ASan is probably to intercept ExitThread and do the deallocation there.

Also, it sounds like small memory leaks are expected when calling `ExitThread`, so maybe we shouldn't worry about this:
> A thread in an executable that is linked to the static C run-time library (CRT) should use _beginthread and _endthread for thread management rather than CreateThread and ExitThread. Failure to do so results in small memory leaks when the thread calls ExitThread. 

https://github.com/llvm/llvm-project/pull/126738