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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast 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.

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