[clang] [Clang] Add support for GCC bound member functions extension (PR #135649)

[Yingwei Zheng via cfe-commits]

dtcxzyw wrote:

> > IIRC this feature is orthogonal to GCC bound member functions. `declcall` with a virtual method just returns a pmf, and we still need to perform vtable lookup at the callsite: https://compiler-explorer.com/z/YrT3nPTEz
> 
> Try `declcall(p->B::virtual_method())`?: https://compiler-explorer.com/z/7d9KPE8zK

We don't know which function is called until the object `p` is provided.
See case 2 in https://discourse.llvm.org/t/rfc-implement-gcc-bound-pmf-in-clang/85951.


https://github.com/llvm/llvm-project/pull/135649