Relands https://github.com/llvm/llvm-project/pull/108375 which had to be
reverted because it was failing on the Windows buildbot. Trying to
reland this with `msvc::no_unique_address` on Windows.
This is motivated by the upcoming refactor of libc++'s
`__compressed_pair` in https://github.com/llvm/llvm-project/pull/76756
As this will require changes to numerous LLDB libc++ data-formatters
(see early draft https://github.com/llvm/llvm-project/pull/96538), it
would be nice to have a test-suite that will actually exercise both the
old and new layout. We have a matrix bot that tests old versions of
Clang (but currently those only date back to Clang-15). Having them in
the test-suite will give us quicker signal on what broke.
We have an existing test that exercises various layouts of `std::string`
over time in `TestDataFormatterLibcxxStringSimulator.py`, but that's the
only STL type we have it for. This patch proposes a new
`libcxx-simulators` directory which will take the same approach for all
the STL types that we can feasibly support in this way (as @labath
points out, for some types this might just not be possible due to their
implementation complexity). Nonetheless, it'd be great to have a record
of how the layout of libc++ types changed over time.
Some related discussion:
*
https://github.com/llvm/llvm-project/pull/97568#issuecomment-2213426804
