.. from the guts of GDBRemoteCommunication to ~top level.

This is motivated by #131519 and by the fact that's impossible to guess
whether the author of a symlink intended it to be a "convenience
shortcut" -- meaning it should be resolved before looking for related
files; or an "implementation detail" -- meaning the related files should
be located near the symlink itself.

This debate is particularly ridiculous when it comes to lldb-server
running in platform mode, because it also functions as a debug server,
so what we really just need to do is to pass /proc/self/exe in a
platform-independent manner.

Moving the location logic higher up achieves that as lldb-platform (on
non-macos) can pass `HostInfo::GetProgramFileSpec`, while liblldb can
use the existing complex logic (which only worked on liblldb anyway as
lldb-platform doesn't have a lldb_private::Platform instance).

Another benefit of this patch is a reduction in dependency from
GDBRemoteCommunication to the rest of liblldb (achieved by avoiding the
Platform dependency).

The function was extremely messy in that it, depending on the set of
arguments, it could either modify the Connection object in `this` or
not. It had a lot of arguments, with each call site passing a different
combination of null values. This PR:
- packs "url" and "comm_fd" arguments into a variant as they are
  mutually exclusive
- removes the (surprising) "null url *and* null comm_fd" code path which
is not used as of https://github.com/llvm/llvm-project/pull/145017
- marks the function as `static` to make it clear it (now) does not
  operate on the `this` object.

Depends on #145017

Listen to gdbserver-port, accept the connection and run `lldb-server gdbserver --fd` on all platforms.
Parameters --min-gdbserver-port and --max-gdbserver-port are deprecated now.

This is the part 2 of #101283.

Fixes #97537.

Replaced `int connection_fd = -1` with `shared_fd_t connection_fd =
SharedSocket::kInvalidFD`.

This is prerequisite for #104238.

…ror() [NFC]

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.

TestPlatformLaunchGDBServer.py runs `ldb-server` w/o parameters
`--min-gdbserver-port`, `--max-gdbserver-port` or `--gdbserver-port`. So
`gdbserver_portmap` is empty and
`gdbserver_portmap.GetNextAvailablePort()` will return 0. Do not call
`portmap_for_child.AllowPort(0)` in this case. Otherwise
`portmap_for_child.GetNextAvailablePort()` will allocate and never free
the port 0 and next call `portmap_for_child.GetNextAvailablePort()` will
fail.

Added few asserts in `GDBRemoteCommunicationServerPlatform::PortMap` to
avoid such issue in the future.

This patch fixes a bug added in #88845. The behaviour is very close to
#97537 w/o parameters `--min-gdbserver-port`, `--max-gdbserver-port` and
`--gdbserver-port`.

Note that StringRef::equals has been deprecated in favor of
operator==(StringRef, StringRef).

This is in preparation to remove the uses of ConstString from
UnixSignals.

Differential Revision: https://reviews.llvm.org/D158209

Summary: For functions that accept an rvalue reference type
parameter, use move to avoid copying the parameter.

These were found when implementing CppCoreGuideline F.18 in
clang-tidy.

Committed on behalf of ccotter (Chris Cotter)

Reviewers: Michael137

Differential Revision: https://reviews.llvm.org/D142824