Adds synthetic children for `std::optional` from MSVC's STL. Most of the
machinery for `std::optional` is already there.

Towards #24834.

When inspecting/printing types from MSVC's STL, LLDB would crash because
it assumes these types were from libstdc++. Specifically,
`std::shared_ptr` and `std::optional` would crash because of a null
pointer dereference. I added a minimal test that tests the types with
C++ helpers for libstdc++ (only tests for crashes).

- Fixes #115216 
- Fixes #120310 

This still has one unresolved discussion: What about MS STL types? This
is https://github.com/llvm/llvm-project/issues/24834, but there was a
bit of discussion in #120310 as well. The main issue is that MSVC's STL
uses the same type names as libstdc++ (i.e. neither uses an inline
namespace like libc++ for some types).

This PR is in continuation of
https://github.com/llvm/llvm-project/pull/136693.

This patch replaces the use of `UINT32_MAX` as the error return value of
`GetIndexOfChildWithName` with `llvm::Expected`.


# Tasks to do in another PR

1. Replace `CalculateNumChildrenIgnoringErrors` with
`CalculateNumChildren`. See [this
comment](https://github.com/llvm/llvm-project/pull/136693#discussion_r2056319358).
2. Update `lldb_private::formatters::ExtractIndexFromString` to use
`llvm::Expected`. See [this
comment](https://github.com/llvm/llvm-project/pull/136693#discussion_r2054217536).
3. Create a new class which carries both user and internal errors. See
[this
comment](https://github.com/llvm/llvm-project/pull/136693#discussion_r2056439608).

The vast majority of `SyntheticChildrenFrontEnd` subclasses provide
children, and as such implement `MightHaveChildren` with a constant
value of `true`. This change makes `true` the default value. With this
change, `MightHaveChildren` only needs to be implemented by synthetic
providers that can return `false`, which is only 3 subclasses.

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.