[clang] [C++20] [Modules] [Itanium ABI] Generate the vtable in the module unit of dynamic classes (PR #75912)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

@rjmccall @dwblaikie 

Since I feel this is a bug we need to fix and I think this patch may not affect non-modules code, I'd like to land this in the end of July (before the next branching of 19) if no more comments came in.

https://github.com/llvm/llvm-project/pull/75912