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

[Richard Smith via cfe-commits]

zygoloid wrote:

> Not sure if anyone has mentioned this elsewhere, but there's a significant problem with the type of the returned function pointer: a normal function pointer doesn't necessarily have the correct calling convention. In practice, this is likely to cause problems on Windows.

The GCC extension [is also broken if a `this` adjustment is needed](https://godbolt.org/z/Ma8GGhK7n), even when the calling convention happens to line up. (And it's not possible to detect this problem locally -- it can happen even if [there is no multiple inheritance visible at the point where the extension is used](https://godbolt.org/z/GGExc1W8b).)

`declcall` doesn't have these problems and seems like a more robust way forward. (It does introduce *some* unnecessary overhead still, though, such as branching on whether the pointer-to-member is for a virtual function and emitting dead code for a virtual call; maybe we could consider some mechanism to declaratively state that a pointer-to-member type is known to not be a virtual pointer if we really care about that.)

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