[clang] [Clang] Implement diagnostics for why `std::is_standard_layout` is false (PR #144161)

[Samarth Narang via cfe-commits]

snarang181 wrote:

@AaronBallman, as per the comment I left below: it is because of the conflicting definitions of the TableGen description of `VirtualFunction`. Will push a resolution soon.


> Needs some manual intervention after the merge. Conflicting redundance in the tablegen descriptions of `VirtualFunction`



https://github.com/llvm/llvm-project/pull/144161