ValueObject is part of lldbCore for historical reasons, but conceptually
it deserves to be its own library. This does introduce a (link-time) circular
dependency between lldbCore and lldbValueObject, which is unfortunate
but probably unavoidable because so many things in LLDB rely on
ValueObject. We already have cycles and these libraries are never built
as dylibs so while this doesn't improve the situation, it also doesn't
make things worse.

The header includes were updated with the following command:

```
find . -type f -exec sed -i.bak "s%include \"lldb/Core/ValueObject%include \"lldb/ValueObject/ValueObject%" '{}' \;
```

…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.

Change the signature of `DWARFExpression::Evaluate` and
`DWARFExpressionList::Evaluate` to return an `llvm::Expected` instead of a
boolean. This eliminates the `Status` output parameter and generally improves
error handling.

This is a proof-of-concept patch that illustrates how to use the
Expected return values to surface rich error messages all the way up
to the ValueObjectPrinter.

This is the final patch in the series that includes
https://github.com/llvm/llvm-project/pull/83501 and
https://github.com/llvm/llvm-project/pull/84219

Change GetNumChildren()/CalculateNumChildren() methods return
llvm::Expected

This is an NFC change that does not yet add any error handling or change
any code to return any errors.

This is the second big change in the patch series started with
https://github.com/llvm/llvm-project/pull/83501

A follow-up PR will wire up error handling.

This reverts commit 99118c809367d518ffe4de60c16da953744b68b9.

Change GetNumChildren()/CalculateNumChildren() methods return
llvm::Expected

This is an NFC change that does not yet add any error handling or change
any code to return any errors.

This is the second big change in the patch series started with
https://github.com/llvm/llvm-project/pull/83501

A follow-up PR will wire up error handling.

In the end this value comes from TypeSystem::GetNumChildren which
returns a uint32_t, so ValueObject should be consistent with that.

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