[llvm] [LLVM][Cygwin] add workaround for blocking connect/accept in AF_UNIX sockets (PR #140353)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> Does that work?

Looks ok here; when merging, one gets to see how it would look in the plaintext form (and one is given a chance to tweak it if necessary).

> BTW, I couldn't find any users of `raw_socket_stream` grepping in llvm-project git worktree, except for the test.

That's odd; perhaps it used to be used somewhere earlier.

https://github.com/llvm/llvm-project/pull/140353