[clang] [Clang][Sema] Fix err_constexpr_virtual_base diagnostic so that it only diagnoses on constructors and destructors (PR #163690)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

if we are doing something special for C++26, I'd like to see tests for it.

https://github.com/llvm/llvm-project/pull/163690