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

[Yingwei Zheng via cfe-commits]

dtcxzyw wrote:

> > We used this extension to improve virtual function calling performance, there are simple and small virtual functions which are frequently called and can not be eliminated and it is in a delegation thus compiler can not optimize.
> > [toplingdb](https://github.com/topling/toplingdb) and [topling-zip](https://github.com/topling/topling-zip) used pmf to pre-bind the functions in several places, like this:
> 
> Can `declcall` (https://wg21.link/p2825) be used for this? It is on track for C++26.

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


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