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

[Shilei Tian via llvm-commits]

shiltian 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.

Please do as it is blocking my build and GCC 7.5.0 is definitely supported by LLVM right now.

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