Desugar any potential references/typedefs before checking
`isStdTemplate`. Previously, the typename might've been:
```
const std::unordered_map<...> &
```
for references. This patch gets the pointee type before grabbing the
canonical type. `GetNonReferenceType` will unwrap typedefs too, so we
should always end up with a non-reference before we get to
`GetCanonicalType`.

https://github.com/llvm/llvm-project/issues/145847

https://github.com/llvm/llvm-project/pull/143501 changes usage of
`__hash_value_type` in libcxx to an empty tag type. This type will no
longer have a definition in DWARF. Currently the LLDB unordered_map
formatter deduces the map's `element_type` by looking at the `__cc_`
member of `__hash_value_type`. But that will no longer work because we
only have its forward declaration. Since what we're really after is the
type that `__hash_value_type` is wrapping, we can just look at the
`__hash_table::value_type` typedef. With
https://github.com/llvm/llvm-project/pull/143501 that will now point to
the `std::pair` element type (which used to be what we got from
`__cc_`).

TBD: need to double-check this works for older layouts. Quick glance at
the code makes me suspicious of cases like `unordered_map<std::pair<int,
int>, int>`

A user ran into an issue where the libc++ `std::unordered_map` formatter
fails because it can't deduce the `element_type`. That happens because
the `node_type` is a forwad declaration. And, in fact, dsymutil stripped
the definition for `std::__1::__hash_node<...>` for a particular
instantiation. While I'm still unclear whether this is a dsymutil bug,
this patch works around said issue by getting the element type from the
`__table_` member.

Drive-by:
- Set the `m_element_type` in `Update`, which is where the other members
are initialized

I don't have a reduced example of this unfortunately. But the crux of
the issue is that `std::__1::__hash_node<...>` only has a forward
declaration in the dsym. Then trying to call `GetTypeTemplateArgument`
on that `CompilerType` fails. And even if the definition was present in
the dsym it seems like we're stopped in a context where the CU only had
a forward declaration DIE for that type and the `node_type` never ends
up being completed with the definition that lives in another CU.

rdar://150813798

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.

There was a bug in both the GNU and libc++ library synthetic child
providers when a typedef was used in the type of the variable. Previous
code was looking at the top level typename to try and determine if
std::unordered_ was a map or set and this failed when typedefs were
being used. This patch fixes both C++ library synthetic child providers
with updated tests.

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%" '{}' \;
```

This patch is in preparation for the `__compressed_pair` refactor in
https://github.com/llvm/llvm-project/pull/76756.

This is mostly reviewable now. With the new layout we no longer need to
unwrap the `__compressed_pair`. Instead, we just need to look for child
members. E.g., to get to the underlying pointer of `std::unique_ptr` we
no longer do,
```
GetFirstValueOfCXXCompressedPair(GetChildMemberWithName("__ptr_"))

```
but instead do
```
GetChildMemberWithName("__ptr_")
```

We need to be slightly careful because previously the
`__compressed_pair` had a member called `__value_`, whereas now
`__value_` might be a member of the class that used to hold the
`__compressed_pair`. So before unwrapping the pair, we added checks for
`isOldCompressedLayout` (not sure yet whether folding this check into
`GetFirstValueOfCXXCompressedPair` is better).

Depends on https://github.com/llvm/llvm-project/pull/97752

This patch changes the way we retrieve the key/value pair in the
`std::unordered_map::iterator` formatter (similar to how we are changing
it for `std::map::iterator` in
https://github.com/llvm/llvm-project/pull/97713, the motivations being
the same).

The old logic was not very easy to follow, and encoded the libc++ layout
in non-obvious ways. But mainly it was also fragile to alignment
miscalculations (https://github.com/llvm/llvm-project/pull/97443); this
would break once the new layout of `std::unordered_map` landed as part
of https://github.com/llvm/llvm-project/issues/93069.

Instead, this patch simply casts the `__hash_iterator` to a
`__node_pointer` (which is what libc++ does too) and uses a
straightforward `GetChildMemberWithName("__value_")` to get to the
key/value we care about.

The `std::unordered_map` already does it this way, so we align the
iterator counterpart to do the same. We can eventually re-use the
core-part of the `std::unordered_map` and `std::unordered_map::iterator`
formatters. But it will be an easier to change to review once both
simplifications landed.