[Clang] Update feature test macros for Clang 18 (#78991)

* Set `__cpp_auto_cast`, as per
https://github.com/cplusplus/CWG/issues/281
* Support `__has_extension(cxx_generalized_nttp)` in C++20 as the
feature isn't stable enough for a feature test macro
* Support `__has_extension(cxx_explicit_this_parameter)` in c++23 as the
feature isn't stable enough for a feature test macro