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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Yes, I'm very eager for this feature, let me investigate the RFC proposal routine.

I would recommend you also include details about how the extension meets our [usual criteria](https://clang.llvm.org/get_involved.html#criteria) when you write the RFC.

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