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%" '{}' \;
```

The the function is doing two fairly different things, depending on how
it is called. While this allows for some code reuse, it also makes it
hard to override it correctly. Possibly for this reason
ValueObjectSynthetic overerides GetChildAtIndex instead, which forces it
to reimplement some of its functionality, most notably caching of
generated children.

Splitting this up makes it easier to move the caching to a common place
(and hopefully makes the code easier to follow in general).

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.

as we don't in general know where the extra data should come from.

Differential Revision: https://reviews.llvm.org/D153657

We used to make a dynamic value that "pretended to be its parent"
but that's hard for some of the more complex ValueObject types, and
it's better in this case just to return no dynamic value.

Differential Revision: https://reviews.llvm.org/D145629

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