`__cpp_explicit_this_parameter` not set in clang-18

[mpusz]

It seems that clang-18 supports explicit object parameter (deducing this) however the feature test macro is not set: https://godbolt.org/z/j6za1evK9

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Mateusz Pusz (mpusz)

It seems that clang-18 [supports explicit object parameter (deducing this)](https://en.cppreference.com/w/cpp/compiler_support/23) however the feature test macro is not set: https://godbolt.org/z/j6za1evK9