Remove workaround for llvm.23435?

There is a workaround in attributes.h:
https://github.com/abseil/abseil-cpp/blob/9eff97861b88999428d1254f95c83d94a2e95944/absl/base/attributes.h
```cpp
// A function-like feature checking macro that accepts C++11 style attributes.
// It's a wrapper around `__has_cpp_attribute`, defined by ISO C++ SD-6
// (https://en.cppreference.com/w/cpp/experimental/feature_test). If we don't
// find `__has_cpp_attribute`, will evaluate to 0.
#if defined(__cplusplus) && defined(__has_cpp_attribute)
// NOTE: requiring __cplusplus above should not be necessary, but
// works around https://bugs.llvm.org/show_bug.cgi?id=23435.
#define ABSL_HAVE_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
#else
#define ABSL_HAVE_CPP_ATTRIBUTE(x) 0
#endif
```
This bug is marked as fixed: https://bugs.llvm.org/show_bug.cgi?id=23435
So, shall this workaround be removed?