[clang] [Clang] Add diagnostic for why std::is_abstract is false (PR #156199)

[Sebastian Proell via cfe-commits]

sebproell wrote:

@shafik @erichkeane Thank you for reviewing.

> Can we add a more detailed summary, at least outlines the various cases you seek to cover e.g. ref, array, union etc

@shafik I hope I understood this correctly and updated the PR description.



https://github.com/llvm/llvm-project/pull/156199