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

[via cfe-commits]

rockeet wrote:

> > > This patch adds support for GCC bound member functions extension: https://gcc.gnu.org/onlinedocs/gcc/Bound-member-functions.html
> > > Related issue: #22495 Closes #82727
> > 
> > 
> > I think this requires an RFC justifying carrying the extension. It's a non-conforming extension that we previously decided we would not implement (#22495 was closed as `WONTFIX`), and there's no real motivation for the feature request listed in #82727.
> > (Personally, I think #82727 should be closed as a duplicate of #22495 unless there's a very strong justification for carrying the extension.)
> 
> #82727 was filed because this extension is used by topling-zip: https://github.com/topling/topling-zip/blob/4c57da837bf1a1612a1524ddf8d258a69f1c3874/src/terark/zbs/blob_store.hpp#L56-L68
> 
> I decided to implement this feature after I read the post by @rockeet :https://zhuanlan.zhihu.com/p/683534962 (in Chinese). TBH I don't think this feature is useful as modern compilers should be smart enough to optimize out redundant vtable lookups.
> 
> cc @rockeet Are you willing to propose an RFC in https://discourse.llvm.org/ to demonstrate its real-world usefulness in ToplingDB?

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

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