[llvm] [llvm][Support] Add UNIX socket support (PR #73603)

[Michael Spencer via llvm-commits]

Bigcheese wrote:

> Am I the only one getting the following error?
> 
> ```
> /home/ac.shilei.tian/Documents/vscode/llvm-project/llvm/lib/Support/raw_ostream.cpp: In static member function ‘static llvm::Expected<llvm::ListeningSocket> llvm::ListeningSocket::createUnix(llvm::StringRef, int)’:
> /home/ac.shilei.tian/Documents/vscode/llvm-project/llvm/lib/Support/raw_ostream.cpp:1040:10: error: could not convert ‘ListenSocket’ from ‘llvm::ListeningSocket’ to ‘llvm::Expected<llvm::ListeningSocket>’
>    return ListenSocket;
>           ^~~~~~~~~~~~
> ```
> 
> Compiler information:
> 
> ```
> ➜  /usr/bin/c++ --version
> c++ (SUSE Linux) 7.5.0
> Copyright (C) 2017 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> ```

Assuming that's gcc 7.5, looks like it doesn't do all of [P0135](https://wg21.link/P0135). That should be easy to change the code to support.

https://github.com/llvm/llvm-project/pull/73603