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

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rockeet 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.

Thank you for your information, I'm new to llvm project, I've written the [[RFC] Implement gcc Bound PMF in clang ](https://discourse.llvm.org/t/rfc-implement-gcc-pmf-in-clang/85951).

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