On Cygwin, UNIX sockets involve a handshake between connect and accept
to enable SO_PEERCRED/getpeereid handling. This necessitates accept
being called before connect can return, but at least the tests in
llvm/unittests/Support/raw_socket_stream_test do both on the same thread
(first connect and then accept), resulting in a deadlock. Add a call to
both places sockets are created that turns off the handshake (and
SO_PEERCRED/getpeereid support).

References:
* https://github.com/cygwin/cygwin/blob/cec8a6680ea1fe38f38001b06c34ae355a785209/winsup/cygwin/fhandler/socket_local.cc#L1462-L1471
* https://inbox.sourceware.org/cygwin/Z_UERXFI1g-1v3p2@calimero.vinschen.de/T/#u

Identified with misc-include-cleaner.

This PR implements `raw_socket_stream::read`, which overloads the base
class `raw_fd_stream::read`. `raw_socket_stream::read` provides a way to
timeout the underlying `::read`. The timeout functionality was not added
to `raw_fd_stream::read` to avoid needlessly increasing compile times
and allow for convenient code reuse with `raw_socket_stream::accept`,
which also requires timeout functionality. This PR supports the module
build daemon and will help guarantee it never becomes a zombie process.

If `::poll` returns and `FD` equals -1, then `ListeningSocket::shutdown`
has been called. So, regardless of any other information that could be
gleaned from `FDs.revents` or `PollStatus`, it is appropriate to return
`std::errc::operation_canceled`. `ListeningSocket::shutdown` copies
`FD`'s value to `ObservedFD` then sets `FD` to -1 before canceling
`::poll` by calling `::close(ObservedFD)` and writing to the pipe.

Commit 87e6f87fe7e343eb656e9b49d30cbb065c086651 adds a call to `::write()`, which may be annotated w/ `warn_unused_result`, leading to `-Wunused-result` failures.

Improvements include
* Enable `ListeningSocket::accept` to timeout after a specified amount
of time or block indefinitely
* Enable `ListeningSocket::createUnix` to handle instances where the
target socket address already exists and differentiate between
situations where the existing file does and does not already have a
bound socket
* Doxygen comments

Functionality added for the module build daemon

---------

Co-authored-by: Michael Spencer <bigcheesegs@gmail.com>

LLVM is inconsistent about how it converts `errno` to `std::error_code`.
This can cause problems because values outside of `std::errc` compare
differently if one is system and one is generic on POSIX systems.

This is even more of a problem on Windows where use of the system
category is just wrong, as that is for Windows errors, which have a
completely different mapping than POSIX/generic errors. This patch fixes
one instance of this mistake in `JSONTransport.cpp`.

This patch adds `errnoAsErrorCode()` which makes it so people do not
need to think about this issue in the future. It also cleans up a lot of
usage of `errno` in LLVM and Clang.

The implementation of raw_string_ostream::write_impl() was moved to
raw_socket_stream.cpp when the raw_socket_ostream support was separated.
This patch moves it back to facilitate disabling socket support in
downstream projects.

Move the implementation of raw_socket_stream from raw_ostream.h/cpp to
raw_socket_stream.h/cpp as requested in #73603.