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 extends the existing register fields support from AArch64 Linux
to AArch64 FreeBSD. So you will now see output like this:
```
(lldb) register read cpsr
    cpsr = 0x60000200
         = (N = 0, Z = 1, C = 1, V = 0, DIT = 0, SS = 0, IL = 0, SSBS = 0, D = 1, A = 0, I = 0, F = 0, nRW = 0, EL = 0, SP = 0) 
```

Linux and FreeBSD both have HWCAP/HWCAP2 so the detection mechanism
is the same and I've renamed the detector class to reflect that.

I have confirmed that FreeBSD's treatment of CPSR (spsr as the kernel
calls it) is similair enough that we can use the same field information.

(see `sys/arm64/include/armreg.h` and `PSR_SETTABLE_64`)

For testing I've enabled the same live process test as Linux
and added a shell test using an existing FreeBSD core file.

Note that the latter does not need XML support because when reading
a core file we are not sending the information via target.xml,
it's just internal to LLDB.

This reverts commit 71c4d186f1cf247f1aa45f4fd1b38f350b68d123.
The reinterpret casts were not needed.

This reverts commit af38530ee29c285f3827e33a41edf27c9c3a6767 due
to a build failure on 32 bit.

So that there is only one function that NativeThreads call,
which takes a siginfo. Everything else is an internal detail.

Reviewed By: labath, JDevlieghere

Differential Revision: https://reviews.llvm.org/D146043

Fix ThreadStopInfo struct to include the signal number for all events.
Since signo was not included in the details for fork, vfork
and vforkdone stops, the code incidentally referenced the wrong union
member, resulting in wrong signo being sent.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D127193

Implement the qXfer:siginfo:read that is used to read the siginfo_t
(extended signal information) for the current thread.  This is currently
implemented on FreeBSD and Linux.

Differential Revision: https://reviews.llvm.org/D117113

Differential Revision: https://reviews.llvm.org/D100547

Differential Revision: https://reviews.llvm.org/D96557