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

About half of our host platform code was implemented in the Platform
class, while the rest was it RemoteAwarePlatform. Most of the time, this
did not matter, as nearly all our platforms are also
RemoteAwarePlatforms. It makes a difference for PlatformQemu, which
descends directly from the base class (as it is local-only).

This patch moves all host code paths into the base class, and marks
PlatformQemu as a "host" platform so it can make use of them (it sounds
slightly strange, but that is consistent with what the apple simulator
platforms are doing). Not all of the host implementations make sense for
this platform, but it can always override those that don't.

I add some basic tests using the platform file apis to exercise this
functionality.

Differential Revision: https://reviews.llvm.org/D122898

To allow us to select a different platform based on where the process is
running, plumb the process host architecture through platform selection.

This patch is in preparation for D121444 which needs this functionality
to tell apart iOS binaries running on Apple Silicon vs on a remote iOS
device.

Differential revision: https://reviews.llvm.org/D121484

All other platforms use GetSupportedArchitectures now.

Module resolution is probably the most complex piece of lldb [citation
needed], with numerous levels of abstraction, each one implementing
various retry and fallback strategies.

It is also a very repetitive, with only small differences between
"host", "remote-and-connected" and "remote-but-not-(yet)-connected"
scenarios.

The goal of this patch (first in series) is to reduce the number of
abstractions, and deduplicate the code.

One of the reasons for this complexity is the tension between the desire
to offload the process of module resolution to the remote platform
instance (that's how most other platform methods work), and the desire
to keep it local to the outer platform class (its easier to subclass the
outer class, and it generally makes more sense).

This patch resolves that conflict in favour of doing everything in the
outer class. The gdb-remote (our only remote platform) implementation of
ResolveExecutable was not doing anything gdb-specific, and was rather
similar to the other implementations of that method (any divergence is
most likely the result of fixes not being applied everywhere rather than
intentional).

It does this by excising the remote platform out of the resolution
codepath. The gdb-remote implementation of ResolveExecutable is moved to
Platform::ResolveRemoteExecutable, and the (only) call site is
redirected to that. On its own, this does not achieve (much), but it
creates new opportunities for layer peeling and code sharing, since all
of the code now lives closer together.

Differential Revision: https://reviews.llvm.org/D113487

There is no reason why this function should be returning a ConstString.

While modifying these files, I also fixed several instances where
GetPluginName and GetPluginNameStatic were returning different strings.

I am not changing the return type of GetPluginNameStatic in this patch, as that
would necessitate additional changes, and this patch is big enough as it is.

Differential Revision: https://reviews.llvm.org/D111877