Reverse-mirrored from toolset update MSVC-PR-650818 targeting MSVC main.

The compiler has implemented a new warning C5291: "'DERIVED': deriving from the base class 'BASE' can cause potential runtime issues due to an ABI bug. Recommend adding a 4-byte data member to the base class for the padding at the end of it to work around this bug." This is emitted in the STL for basic_iostream and basic_ostringstream, which each derive from basic_ostream, when we try to self-build MSVC with a sufficiently updated compiler.

Whatever this ABI bug is, it's frozen in our layout of basic_ostream. Since we can't do anything about it until vNext, I'm globally suppressing it in the STL, with a TRANSITION, ABI comment.

I considered fine-grained suppression around basic_iostream and basic_ostringstream, in case we wanted to leave this warning active for newly added internal classes, but the risk seems low; we virtually never use multiple inheritance in internal classes. To avoid risk for the internal toolset update, I went with the global suppression.

The compiler warning was implemented on 2025-06-03 by MSVC-PR-640189, where @xiangfan-ms said:

When we determine the size of a class when it is used as a base class, we don't take the alignment of the class into consideration. This means we may incorrectly insert members from the derived class to the base class, even though the rest of the compiler doesn't expect this. This PR adds an off-by-default warning to help detect such cases.

I noticed the "off-by-default" part after making this change in MSVC; I'll follow up later to add our usual "(/Wall)" comment. Fortunately, this means that it won't cause warnings for users in our supported /W4 scenarios.