diagnostics: add experimental SARIF JSON-RPC notifications for IDEs [PR115970]

diagnostics: add experimental SARIF JSON-RPC notifications for IDEs [PR115970]

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3358r0.html#msvc
describes a feature of Visual Studio 2022 version 17.8. which can send its
diagnostics in SARIF form to a pipe when setting the environment variable
SARIF_OUTPUT_PIPE:
  https://learn.microsoft.com/en-us/cpp/build/reference/sarif-output?view=msvc-170#retrieving-sarif-through-a-pipe

The precise mechanism above involves Windows-specific details (windows pipes
and HANDLEs).

The following patch implements an analogous feature for GCC, using Unix
domain sockets rather than the Windows-specific details.

With this patch, GCC's cc1, cc1plus, etc will check if
EXPERIMENTAL_SARIF_SOCKET is set in the environment, and if so,
will attempt to connect to that socket.  It will send a JSON-RPC
notification to the socket for every diagnostic emitted.  Like the
MSVC feature, the diagnostics are sent one-at-a-time as SARIF
"result" objects, rather than sending a full SARIF "log" object.

The patch includes a python test script which runs a server.
Tested by running the script in one terminal:

$ ../../src/contrib/sarif-listener.py
listening on socket: /tmp/tmpjgts0u0i/socket

and then invoking a build in another terminal with the envvar
set to the pertinent socket:

$ EXPERIMENTAL_SARIF_SOCKET=/tmp/tmpjgts0u0i/socket \
    make check-gcc RUNTESTFLAGS="analyzer.exp=*"

and watching as all the diagnostics generated during the build
get sent to the listener.

The idea is that an IDE ought to be able to create a socket and
set the environment variable when invoking a build, and then listen
for all the diagnostics, without needing to manually set build flags
to inject SARIF output.

This feature is experimental and subject to change or removal
without notice; I'm adding it to make it easier for IDE developers to
try it out and give feedback.

contrib/ChangeLog:
	PR diagnostics/115970
	* sarif-listener.py: New file.

gcc/ChangeLog:
	PR diagnostics/115970
	* diagnostics/sarif-sink.cc: Include <sys/un.h> and <sys/socket.h>.
	(sarif_builder::end_group): Update comment.
	(sarif_sink::on_end_group): Drop "final".
	(class sarif_socket_sink): New subclass.
	(maybe_open_sarif_sink_for_socket): New function.
	* diagnostics/sarif-sink.h: (maybe_open_sarif_sink_for_socket):
	New decl.
	* doc/invoke.texi (EXPERIMENTAL_SARIF_SOCKET): Document new
	environment variable.
	* toplev.cc: Define INCLUDE_VECTOR. Add include of
	"diagnostics/sarif-sink.h".
	(toplev::main): Call
	diagnostics::maybe_open_sarif_sink_for_socket.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>