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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast 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

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