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.

PT_STOP was introduced with NetBSD 9.0.

This is a follow-up to D116372, which had a rather unfortunate side
effect of making the processing of a single SIGCHLD quadratic in the
number of threads -- which does not matter for simple applications, but
can get really bad for applications with thousands of threads.

This patch fixes the problem by implementing the other possibility
mentioned in the first patch -- doing waitpid(-1) centrally and then
routing the events to the correct process instance. The "uncollected"
threads are held in the process factory class -- which I've renamed to
Manager for this purpose, as it now does more than creating processes.

Differential Revision: https://reviews.llvm.org/D146977

Add a new SaveCore() process method that can be used to request a core
dump.  This is currently implemented on NetBSD via the PT_DUMPCORE
ptrace(2) request, and enabled via 'savecore' extension.

Protocol-wise, a new qSaveCore packet is introduced.  It accepts zero
or more semicolon-separated key:value options, invokes the core dump
and returns a key:value response.  Currently the only option supported
is "path-hint", and the return value contains the "path" actually used.
The support for the feature is exposed via qSaveCore qSupported feature.

Differential Revision: https://reviews.llvm.org/D101285

Remove hardcoded platform list for QPassSignals, qXfer:auxv:read
and qXfer:libraries-svr4:read and instead query the process plugin
via the GetSupportedExtensions() API.

Differential Revision: https://reviews.llvm.org/D101241

Differential Revision: https://reviews.llvm.org/D100554

Differential Revision: https://reviews.llvm.org/D98822

This commit has caused the following tests to be flaky:
TestThreadSpecificBpPlusCondition.py
TestExitDuringExpression.py

The exact cause is not known yet, but since both tests deal with
threads, my guess is it has something to do with the tracking of
creation of new threads (which the commit touches upon).

This reverts the following commits:
d01bff8cbdc98fb8751f7bf10af19b47ae5c445d,
ba62ebc48e8c424ce3a78ba01acda679d536dd47,
e761b6b4c58d4f7ae1073d925d7cb321d68ee93a,
a345419ee03095c8cdfbe1c2728467c4da8fa0a4.