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).
9e9b1178ca