[llvm] Fix thread handle leak on Windows (PR #156854)

[Daniel Paoliello via llvm-commits]

https://github.com/dpaoliello approved this pull request.

LGTM - `thread::join` clears the handle after calling this, so we are definitely leaking at the moment:
https://github.com/llvm/llvm-project/blob/8bdaa12b7ca4450d0f29b93646f1be27c0d717bd/llvm/include/llvm/Support/thread.h#L144-L147

https://github.com/llvm/llvm-project/pull/156854