As part of https://github.com/llvm/llvm-project/pull/143177, I moved the
non-libc++ specific formatting of `std::string`s out to `CxxStringTypes`
as MSVC's STL `std::string` can also be thought of a pointer+size pair.
I named this kind of string "string buffer".

This PR picks that change, so the MSVC PR can be smaller.
Unfortunately, libstdc++'s `std::string` does not fit this (it also uses
a different string printer function).

This resolves two FIXMEs in the libc++ tests, where empty u16 and u32
strings didn't have any prefix (u/U).

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).

This patch pushes the error handling boundary for the GetBitSize()
methods from Runtime into the Type and CompilerType APIs. This makes it
easier to diagnose problems thanks to more meaningful error messages
being available. GetBitSize() is often the first thing LLDB asks about a
type, so this method is particularly important for a better user
experience.

rdar://145667239

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.

This has two changes:
1. Set show value for libcxx and libstdcxx summary provider. This will
print the pointer value for both pointer type and reference type.
2. Remove pointer value printing in libcxx container summary.

Discussion:

https://discourse.llvm.org/t/lldb-hides-raw-pointer-value-for-libcxx-and-libstdcxx-pointer-types-in-summary-string/84226

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).

Similar to how we moved the `std::map::iterator` formatter in
https://github.com/llvm/llvm-project/pull/97687, do the same for
`std::unordered_map::iterator`.

Again the `unordered_map` and `unordered_map::iterator` formatters try
to do very similar things: retrieve data out of the map. The iterator
formatter does this in a fragile way (similar to how `std::map` does it,
see https://github.com/llvm/llvm-project/pull/97579). Thus we will be
refactoring the `std::unordered_map::iterator` in upcoming patches.
Having it in `LibCxxUnorderedMap` will allow us to re-use some of the
logic (and we won't have to repeat some of the clarification comments).

The two formatters follow very similar techniques to retrieve data out
of the map. We're changing this for `std::map` in
https://github.com/llvm/llvm-project/pull/97579 and plan to change it in
the same way for the iterator formatter. Having them in the same place
will allow us to re-use some of the logic (and we won't have to repeat
some of the clarification comments).

The nullness is already checked a few lines before this.