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

[via cfe-commits]

rockeet 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, so we use pmf to bound the virtual function, like this:
```c++
#if defined(_MSC_VER) || defined(__clang__)
  #define IF_BOUND_PMF(Then, Else) Else
#else
  #define IF_BOUND_PMF(Then, Else) Then
#endif
  void SetSubReader(const ToplingZipSubReader* sub) {
    subReader_ = sub;
    iter_ = sub->index_->NewIterator();
    store_ = sub->store_.get();
    get_record_append_ = store_->m_get_record_append_CacheOffsets;
   #if defined(_MSC_VER) || defined(__clang__)
   #else
    iter_next_ = (IterScanFN)(iter_->*(&COIndex::Iterator::Next));
    iter_prev_ = (IterScanFN)(iter_->*(&COIndex::Iterator::Prev));
   #endif
    tag_rs_kind_ = sub->tag_rs_kind_;
  }

  inline bool IndexIterInvokeNext() {
    return IF_BOUND_PMF(iter_next_(iter_), iter_->Next());
  }
  inline bool IndexIterInvokePrev() {
    return IF_BOUND_PMF(iter_prev_(iter_), iter_->Prev());
  }
```
In which `iter_->Next()` is a small virtual function for different index type, we save the pmf as a member, thus reduced the virtual function calling overhead.


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