In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.

The LLDB client has support for structured data plugins, but lldb-server
doesn't have corresponding support for it. This patch adds the missing
functionality in LLGS for servers to register their supported plugins
and send corresponding async messages.

This patch removes all of the Set.* methods from Status.

This cleanup is part of a series of patches that make it harder use the
anti-pattern of keeping a long-lives Status object around and updating
it while dropping any errors it contains on the floor.

This patch is largely NFC, the more interesting next steps this enables
is to:
1. remove Status.Clear()
2. assert that Status::operator=() never overwrites an error
3. remove Status::operator=()

Note that step (2) will bring 90% of the benefits for users, and step
(3) will dramatically clean up the error handling code in various
places. In the end my goal is to convert all APIs that are of the form

`    ResultTy DoFoo(Status& error)
`
to

`    llvm::Expected<ResultTy> DoFoo()
`
How to read this patch?

The interesting changes are in Status.h and Status.cpp, all other
changes are mostly

` perl -pi -e 's/\.SetErrorString/ = Status::FromErrorString/g' $(git
grep -l SetErrorString lldb/source)
`
plus the occasional manual cleanup.

This patch replaces (llvm::|)Optional< with std::optional<.  I'll post
a separate patch to clean up the "using" declarations, #include
"llvm/ADT/Optional.h", etc.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

This patch adds #include <optional> to those files containing
llvm::Optional<...> or Optional<...>.

I'll post a separate patch to actually replace llvm::Optional with
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

Introduce a new %Stdio notification category and use it to send process
output asynchronously when running in non-stop mode.  This is an LLDB
extension since GDB does not use the 'O' packet for process output,
just for replies to 'qRcmd' packets.

Using the async notification mechanism implies that only the first
output packet is sent immediately to the client.  The client needs
to request subsequent notifications (if any) using the new vStdio packet
(that works pretty much like vStopped for the Stop notification queue).

The packet handler in lldb-server tests is updated to handle the async
stdio packets in addition to the regular O packets.  However, due
to the implications noted above, it can only handle the first output
packet sent by the server.  Subsequent packets need to be explicitly
requested via vStdio.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D128849

Extend vCont function to support resuming a process with an arbitrary
PID, that could be different than the one selected via Hc (or no process
at all may be selected).  Resuming more than one process simultaneously
is not supported yet.

Remove the ReadTid() method that was only used by Handle_vCont(),
and furthermore it was wrongly using m_current_process rather than
m_continue_process.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D127862

Implement the support for the vKill packet.  This is the modern packet
used by the GDB Remote Serial Protocol to kill one of the debugged
processes.  Unlike the `k` packet, it has well-defined semantics.

The `vKill` packet takes the PID of the process to kill, and always
replies with an `OK` reply (rather than the exit status, as LLGS does
for `k` packets at the moment).  Additionally, unlike the `k` packet
it does not cause the connection to be terminated once the last process
is killed — the client needs to close it explicitly.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D127667

Implement the support for %Stop asynchronous notification packet format
in LLGS.  This does not implement full support for non-stop mode for
threaded programs -- process plugins continue stopping all threads
on every event.  However, it will be used to implement asynchronous
events in multiprocess debugging.

The non-stop protocol is enabled using QNonStop packet.  When it is
enabled, the server uses notification protocol instead of regular stop
replies.  Since all threads are always stopped, notifications are always
generated for all active threads and copied into stop notification
queue.

If the queue was empty, the initial asynchronous %Stop notification
is sent to the client immediately.  The client needs to (eventually)
acknowledge the notification by sending the vStopped packet, in which
case it is popped from the queue and the stop reason for the next thread
is reported.  This continues until notification queue is empty again,
in which case an OK reply is sent.

Asychronous notifications are also used for vAttach results and program
exits.  The `?` packet uses a hybrid approach -- it returns the first
stop reason synchronously, and exposes the stop reasons for remaining
threads via vStopped queue.

The change includes a test case for a program generating a segfault
on 3 threads.  The server is expected to generate a stop notification
for the segfaulting thread, along with the notifications for the other
running threads (with "no stop reason").  This verifies that the stop
reasons are correctly reported for all threads, and that notification
queue works.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D125575