This is the first useful patch in the series related to enabling
`PTRACE_SEIZE` for processes Coredumping. In order to make the decision
if we want to seize or attach, we need to expose that in processinfo.
Which we acquire by reading it from `/proc/pid/status`

Note that in status it is `CoreDumping` not `Coredumping`, so I kept
with that, even if I prefer `Coredumping`

This PR is in reference to porting LLDB on AIX.

Link to discussions on llvm discourse and github:

1. https://discourse.llvm.org/t/port-lldb-to-ibm-aix/80640
2. https://github.com/llvm/llvm-project/issues/101657
The complete changes for porting are present in this draft PR:
https://github.com/llvm/llvm-project/pull/102601

- Added changes to make the common host support functions under
`Host/posix` for unix-like system.
Also, created the `unittests/Host/posix/` to test the hostInfo & support
functions for unix-like system.
- Added changes to get the host information for AIX. (GetProcessInfo()) 
(Information like : executable path, arch, process status etc.)

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.

As reported in https://github.com/llvm/llvm-project/issues/89710, the %s
code used for `comm` could and probably does, overflow the buffer.
Likely we haven't seen it cause problems because the following data is
overwritten right afterwards.

Also scanf isn't a great choice here as this `comm` can include many
characters that might trip up %s.

We don't actually use `comm`, so parse but don't store it so we're not
overflowing anything.

To implement SaveCore for elf binaries we need to populate some
additional fields in the prpsinfo struct. Those fields are the nice
value of the process whose core is to be taken as well as a boolean flag
indicating whether or not that process is a zombie. This commit adds
those as well as tests to ensure that the values are consistent with
expectations

This adds some additional bits into a ProcessInfo structure that will be
of use in filling structs in an elf core file. This is a demand for
implementing process save-core

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